Tutorials

Getting Started with GIS

 The web is a major resource for data and information Geographic Information Systems (GIS). Spatial data are the backbone of a GIS, and the web is a limitless source of such data. This fact is both a blessing and a curse, because one needs to remember that a GIS is only as good as the data in it. Thus, going out on the web, locating a vast amount of data, and downloading it without a plan and a scheme to verify these data is NOT the way to construct a GIS.

Geographic Information Systems

The web is a major resource for data and information Geographic Information Systems (GIS). Spatial data are the backbone of a GIS, and the web is a limitless source of such data. This fact is both a blessing and a curse, because one needs to remember that a GIS is only as good as the data in it. Thus, going out on the web, locating a vast amount of data, and downloading it without a plan and a scheme to verify these data is NOT the way to construct a GIS. However, with a plan and an understanding of the data required for a specific project, the web is certainly an incredible source of information. In addition, there are many educational sites, tutorials, and overviews of data and systems that have been built by government agencies, companies, and research programs. However, the volume and variety of this information is often overwhelming. Below we have selected some key sites targeting those wanting to get started understanding and building a GIS. Our intent is to provide beginners with a manageable list of particularly useful sites. These are our picks and we would be happy to receive comments and suggestions regarding other useful sites.

 Introduction

There is a lot more to becoming proficient in the use of GIS than learning to manipulate computer programs such as ArcView and ArcInfo, but proficiency with GIS software is essential. However, learning how to move data around a computer system and perform simple tasks such as reformatting is also essential. For example, there is a considerable amount of data available on the web, but little of it can be input directly into GIS software without manipulation.

 The data base is the most important element of a GIS, and it takes significant discipline expertise to choose wisely from existing data sources and to generate quality data. In addition, its takes discipline and expertise to conduct ground truth studies.

Most data are inherently either raster or vector in nature. DEMs (digital elevation models) and images are the most common raster data and are mostly used as the first layer in a GIS (i.e., the layer on which vector data are overlain). Vector data are defined by either points, lines, or polygons. Links between vector data and tabular information are the key to differentiating between a GIS and computer cartography.

 The generation of high quality graphic products requires a working knowledge of map projections and the basics of geodesy. Multiple conversions between projections, coordinate systems, and reference ellipsoids are an inevitable part of the production of a graphic product from a GIS.

 Sometimes simply displaying data in an effective way geographically provides the information desired. However, further analysis of the data is often needed. This analysis may involve straightforward statistics or simple relations such as less than, greater than, equals, change over time, etc. Gridding point data to create a contour map or a surface using a TIN (triangulated irregular network) is an example of a more sophisticated mathematical operation. The input of data into a groundwater flow modeling program and displaying the results is another.

 Vector or Raster ?

A basic issue that must be understood from a variety of perspectives is the difference between vector and raster data. Vector data, can be defined by points, lines, or polygons (areas) and are efficient in terms of computer resources required for storage and ease of plotting. Objects that one draws with a program such as Illustrator are stored as vectors. Raster data are stored as regular grids and are usually displayed as images. A scanned image is stored as raster information and Photoshop is an example of a program designed to manipulate such data. In the simplest case, the value at pixel location is either black (0) or white (1) and requires storage of only 1 bit of information. Shades of gray are often represented by values ranging from 0 (black) to 255 (white). The 256 possible values ranges over 2 to the 8th power or 8 bits. Thus, a gray-scale image requires about 8 times as much storage as a black and white one! Color requires 3 bands (or scans) of data so that red, green, and blue values can be created and plotted. With each band representing 8 bits of variation, the color image would require 24 bits (over 16 million) of storage for each pixel. GIS texts tend to discuss this issue without sufficient emphasis on its impact on hardware and software resources.

 Some Tutorials to Get You Started

  1. ERSI Virtual Campus - a source of on-line courses a source of on-online courses

  2. Overview of GIS - The Geographer's Craft Project - Peter Dana - University of Texas at Austin

  3. U. S. Geological Survey - GIS Tutorial

  4. University of British Columbia - Department of Geography

  5. The GIS Primer - David J. Buckley - Pacific Meridian Resources, Inc.

Resources

 

Classifying Data

On maps, we usually think of information being represented as points (i.e., benchmarks, sample locations, drill hole locations), lines, (property boundaries, lineaments, roads), or areas (lakes, mines, outcrops). However, geoscience data are more complicated than this and involve points, lines, areas, surfaces, volumes, and temporal variations. One way to think about data is in terms of its dimensionality.

On maps, we usually think of information being represented as points (i.e., benchmarks, sample locations, drill hole locations), lines, (property boundaries, lineaments, roads), or areas (lakes, mines, outcrops). However, geoscience data are more complicated than this and involve points, lines, areas, surfaces, volumes, and temporal variations. One way to think about data is in terms of its dimensionality.

Point data are tabular in form and can be viewed in a spread sheet or as symbols and/or numbers on a map. Such data have no spatial dimension (just a location) so we can think of them as being 0-D. To plot such information, we usually instruct the computer to find the location and plot a symbol and/or number.

Data that are represented by lines alone are 1-D, because a line is infinitely narrow. A contact between two geologic bodies is a good example. Different types of roads may be represented by different types of lines and thus be 1-D. However, a road may take on a width in a GIS as we differentiate between features such as Interstate highways and dirt roads and consider the actual land area they cover. What about lines that are not straight? The main point is that they should be thought of as 1-D if they have no width. To plot such information, we usually instruct the computer to connect two points with a line of a certain width or pattern. Curves are usually just a series of short line segments.

 0-D and 1-D data would normally be represented as vectors in a GIS.

 Data that are represented by areas are 2-D. Outcrops on geologic maps and soil maps are common examples. Political features such as outlines of counties, states, countries, etc. are another. The most efficient way to store such data is as a vector defining the outline so plotting only involves drawing the outline and perhaps using a fill pattern. We would normally think of data such as aerial photographs, Landsat TM images, or shaded relief depictions of DEMs as being 2-D but raster in nature. Such data require considerable storage (and often processing), and plotting requires color or shades of gray. Professional results require a high-quality plotter. Some GIS software has limited capability to handle such data.

Data sets that we normally represent as contour maps (elevation data, gravity readings, depths to horizons in drill holes) are not exactly 2-D or 3-D in nature. Thus, we can think of them as being 2 1/2 - D. This term can be a little abstract, but the idea is to classify data that we plot as a surface (in color or as contours or both) without it representing a volume. Such data are usually stored as discrete values (x, y, z - elevation, gravity values, depth), and the process whereby these values are interpolated to define a surface which can be contoured is a major, but often under-appreciated, issue.

 Data sets that represent volumes are 3-D. Examples would be the outline of an ore body, a salt dome, or a contaminate plume. Such features cannot be plotted on map except by projecting their outline onto the surface of a map. Most GIS software would require access to a special 3-D plotting package to display such data.

 In the past few years, it has been common to refer to data which include changes with time as being 4-D. This practice is most common in the petroleum industry where changes in a reservoir over time are monitored. However, the monitoring of a groundwater resource, the search for temporal changes on images, or tracking the movement of pollutants can be thought of this way. In plotting such data, the goal is to display the changes.

As we think of spatial objects that are defined with some data structure, we can classify them in other ways:

  • Natural vs. man-made (or imposed)

  • Sampling limited vs. definition limited (salt dome vs. ore body)

  • Irregular vs. regular (scattered samples vs. grid)

 Projections

Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps. Before the digital data can be analyzed, they may have to undergo other manipulations - projection conversions, for example - that integrate them into a GIS.

Background Information on Map Projections

Map information in a GIS must be manipulated so that it registers, or fits, with information gathered from other maps. Before the digital data can be analyzed, they may have to undergo other manipulations - projection conversions, for example - that integrate them into a GIS.

Projection is a fundamental component of map making. A projection is a mathematical means of transferring information from the Earth's three dimensional curved surface to a two-dimensional medium - paper or a computer screen. Different projections are used for different types of maps because each projection is particularly appropriate to certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.

Since much of the information in a GIS comes from existing maps, a GIS uses the processing power of the computer to transform digital information, gathered from sources with different projections to a common projection.  

Scale

Geoscientists are generally familiar with the concept of scale, but it is a key issue that effects data integrity. A confusing fact is that regional maps at scales such 1:1,000,000 are referred to as small scale (a given distance on the ground is a small distance on the map) while detailed maps such as the 1:24,000 topographic series (7 1/2 minute quads) are referred to a large scale.

Scale - A Very Important Consideration

Definition from Bonham-Carter :

 The ratio of the distance on a map, photograph, or image to the corresponding distance on the ground, all in the same units (i.e., 1:24,000; 1:100,000; 1;250,000; 1:500,000; 1:1,000,000)

 Geoscientists are generally familiar with the concept of scale, but it is a key issue that effects data integrity. A confusing fact is that regional maps at scales such 1:1,000,000 are referred to as small scale (a given distance on the ground is a small distance on the map) while detailed maps such as the 1:24,000 topographic series (7 1/2 minute quads) are referred to a large scale. In the U.S., geographical and geological data products provided by government agencies are usually linked to the scale of the map from which they were derived. This fact is all to often overlooked by GIS users and is an important way to document and communicate the accuracy of the data. It is also a situation where the terms accuracy, precision, and resolution are used in confusing and inconsistent ways It is important to remember that these data products are usually derived by digitizing (by hand or by scanning) information from the corresponding map and thus can be no more accurate in a spatial sense than as they appear on the map. Products of the U.S. Geological Survey that geoscientists use regularly are vector (digital line graph - DLG) and digital elevation grids (digital elevation model - DEM) digitized from 1:24,000, 1:100,000, and 1:250,000 scale maps. As one progresses from 1:250,000 to 1:100,000 to 1:24,000 data, the accuracy and the volume increase. This is a good example of where the user needs to know "how good is good enough."

Example of Vector/Point Data

 The location of an object such as a well on a 1:500,000 is digitized and the practical accuracy of this location is 100th of an inch. Since 1 inch on the map represents 500,000 inches on the ground, the precision of the location of the object is about +/- 5000 inches (417 ft). An additional consideration is the fact that the position as plotted on the map is imperfect and that the map is usually a piece of paper that is not scale stable (i.e., it is warped by the printing process - wet ink on dry paper; humidity, wear and tear - coffee spills, perspiration, folds, tears, etc.). These considerations mean that the accuracy of this location may be as bad as +/- 1000 ft. On a 1:24,000 scale map, the same procedure leads to a precision of +/- 240 inches (20 ft). These numbers should be compared to GPS measurements where one generates fundamentally new data that can easily be accurate to +/- 2 inches.

Example of Grid/Raster Data

Digital Elevation Models (DEM) are examples of where spatial resolution and vertical accuracy cause confusion. The spacing between grid points in a DEM should be thought of as a metric measuring resolution (i.e., 30 m in the case of a 1:24,000 scale DEM). A separate question is the accuracy of the elevation values in the grid. The USGS standards for DEMs are rather complicated, but the elevation accuracy of a DEM can intuitively be linked to the contour interval of the topographic map from which it is derived. The national standard is that 90% of the points in the DEM are correct to 1/2 the contour interval. Again, these numbers should be compared to GPS measurements where one generates fundamentally new data that can easily be accurate to +/- 2 inches.

Links  

Geodetic Datums and Coordinate Systems

Error, Accuracy, and Precision

Background Information on the Global Positioning System and Its Applications

USGS Geographic Information

Computer Cartography

  • Generic Mapping Tools
    An excellent free program developed by Paul Wessel and Walter H. F. Smith. - University of Hawai

If You Want to Learn More About GIS and Related Topics, Here are Some Good Places to Start

Digital Elevation Models

DEM is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The standard DEM consists of a regular array of elevations cast on a designated coordinate projection system. The DEM data are stored as a series of profiles in which the spacing of the elevations along and between each profile is in regular whole number intervals.

USGS, UDGC, and NIMA Digital Elevation Models
 

Data Sets  

The National Geophysical Data Center (NOAA) provides topographic data on a global basis.

ETOPO5

Earth Topography- 5 Minute digital average land and sea floor elevations were assembled from several uniformly gridded data bases into a worldwide gridded dataset with a grid spacing of 5 minutes of latitude by 5 minutes of longitude.

Oceanic Bathymetry

Oceanic bathemetry was compiled by the U.S. Naval Oceanographic Office, and revised by them in 1987. The land elevations of the conterminous United States, Japan, and Western Europe were compiled from gridded data supplied by the Defense Mapping Agency. Land elevations on New Zealand came from the Department of Scientific and Industrial Research of New Zealand; land elevations for Australia were supplied by the Bureau of Mineral Resources of Australia. The balance of the land elevations were interpolated from 10-minute gridded modal height data from the U.S. Navy Fleet Numerical Oceanographic Center.

Terrain Base CD-ROM

National Geophysical Data Center has available a collection of public domain digital terrain models (DTMs) for the world on CD. This collection, called the Terrain Base CD-ROM, includes an improved 5-minute DTM of land and ocean values for the entire world. Terrain Base includes ten additional sources of data that provide a substantial improvement in overall quality of DTMs.

 

GLOBE

An ongoing project at the NGDC. The Global Land One-kilometer Base Elevation (GLOBE) Project is an international effort to develop a best-available global DEM on a 1-kilometer grid. The general aims of GLOBE are:

  1. Develop a 1-km global DEM, by including the best available data sets and by encouraging specialists to participate in production and review of the data. The GLOBE DEM is being made available to the worldwide research community. Several prototypes are on the World Wide Web. Final versions are likely to be available on the World Wide Web and CD-ROM. There are two versions of GLOBE:

    • Best Available Data (BAD), even if some data are restricted from general distribution. This version would be distributed in accordance with whatever agreement is negotiated with contributors of various copyright data sets. High-quality restricted data may improve the usability of those data for people having permission to use such data. Currently, there is one copyright data set offered to GLOBE for Australia.
    • Globally Only Open-access Data (GOOD). This is the primary aim of GLOBE; to produce an unrestricted DEM that is the most useful database to everyone.
  2. Strengthen international collaboration in the development of research-quality digital global data sets. Advance technical and cultural capabilities for international collaboration in the development of such data.

  3. Strengthen social awareness of the need for optimal quality high-resolution global topographic information, including the provision of a focus for the timely release of currently restricted terrain data sets.

  4. Supply a "pathfinder" data set to the Earth observation community.

  5. Develop a data structure (nested multi-resolution grid system) useful for future enhancements such as might come from future topographic satellite missions.

  6. Give the Committee on Earth Observation Satellites Subgroup on Auxiliary Data Sets (CEOS-ADS) a prototype in cooperatively improving vital data.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 NIMA

In support of military applications, the National Imagery and Mapping Agency (NIMA) has developed a standard digital dataset (Digital Terrain Elevation Data (DTED®) Level 0) which may be of value to scientific, technical, and other communities. This DTED® product is a uniform matrix of terrain elevation values which provides basic quantitative data for systems and applications that require terrain elevation, slope, and/or surface roughness information.

National Imagery and Mapping Agency (NIMA)

Digital Terrain Elevation Data (DTED®) Level 0

30 Arc Second Terrain Data

In support of military applications, the National Imagery and Mapping Agency (NIMA) has developed a standard digital dataset (Digital Terrain Elevation Data (DTED®) Level 0) which may be of value to scientific, technical, and other communities. This DTED® product is a uniform matrix of terrain elevation values which provides basic quantitative data for systems and applications that require terrain elevation, slope, and/or surface roughness information. DTED® Level 0 elevation post spacing is 30 arc second (nominally one kilometer). In addition to this discrete elevation file, a separate binary file provides the minimum, maximum, and mean elevation values computed in 30 arc second square areas (organized by one degree cell). Finally, DTED® Level 0 contains the NIMA Digital Mean Elevation Data (DMED) providing minimum, maximum, and mean elevation values and standard deviation for each 15 minute by 15 minute area in a one degree cell. This initial prototype release is a "thinned" data file extracted from the NIMA DTED® Level 1 holdings where available and from the elevation layer of NIMA VMAP Level 0 to complete near world-wide coverage. The current DTED® Level 0 and subsequent releases will be updated consistent with established NIMA production maintenance procedures.

Support from select international mapping organizations was instrumental in the generation of the Level 0 dataset. The following nations have contributed data to this effort: Belgium, Denmark, France, Germany, Italy, Netherlands, Norway, Spain, and the United Kingdom. In many cases, higher resolution terrain data of the above listed nations is available for public sale. If you are interested in obtaining such data over any of the above listed countries, please contact the civil mapping authorities of the individual nation directly.


Digital Line Graphs

A major type of data in a GIS is a Digital Line Graph (DLG). A DLG is a digital representations by points, lines and areas of planimetric information derived from 7.5- and 15-minute scale topographic quadrangle maps (large-scale); 30- by 60-minute intermediate scale maps; and 1:2 million-scale (small scale) sectional National Atlas maps. All DLG data distributed by the United States Geological Survey (USGS) are DLG-Level 3 (DLG-3), which means the data contain a full range of attribute codes, have full topological structuring, and have passed certain quality-control checks.

Digital Line Graphs (DLG)

Modified from the U.S. Geological Survey

A major type of data in a GIS is a Digital Line Graph (DLG). A DLG is a digital representations by points, lines and areas of planimetric information derived from 7.5- and 15-minute scale topographic quadrangle maps (large-scale); 30- by 60-minute intermediate scale maps; and 1:2 million-scale (small scale) sectional National Atlas maps. All DLG data distributed by the United States Geological Survey (USGS) are DLG-Level 3 (DLG-3), which means the data contain a full range of attribute codes, have full topological structuring, and have passed certain quality-control checks.

These data are available at little or no cost, and many can be downloaded over the internet.

Summary

  • Large-scale DLG data files are available for the U.S. Public Land Survey, boundaries, transportation, hydrography and in some areas, hypsography. They are produced in 7.5-minute by 7.5-minute units which correspond to USGS 1:20,000, 1:24,000, and 1:25,000-scale topographic quadrangle maps. The unit sizes of 1:63,360-scale Alaska quadrangles vary depending on the latitudinal location of the unit.
  • Intermediate (1:100,000 scale) DLG data files that cover transportation and hydrography, are available for all States except Alaska. Intermediate -scale DLG data are sold in 30-minute by 30-minute units which correspond to the east half or west half of USGS 30- by 60-minute ,1:100,000-scale topographic quadrangle maps. Each 30-minute unit is produced and distributed as four 15- by 15-minute cells, except in high-density areas, where the 15-minute cells may be subdivided into four 7.5-minute cells.
  • Small-scale DLGs of the National Atlas of the United States at 1:2 million scale are available for boundaries, transportation and hydrography. These small-scale DLG data correspond to USGS 1:2 million-scale sectional maps of the National Atlas of the United States of America and are sold in 21 units. Fifteen sections cover the conterminous United States, five cover Alaska and one covers Hawaii. These sectional DLGs are usually sold in multi-State units. However, some may cover only one State or a portion of a State.

Intermediate-Scale DLGs

Intermediate-scale DLGs are derived from USGS 1:100,000-scale 30- by 60- minute quadrangle maps. If these maps are not available, Bureau of Land Management planimetric maps at 1:100,000 scale are used, followed by archival compilation materials.

Intermediate-scale DLGs are sold in 30- by 30-minute units that correspond to the east or west half of USGS 30- by 60-minute 1:100,000-scale topographic quadrangle maps. Each 30-minute unit is produced and distributed as four 15- by 15-minute cells, except in high-density areas, where the 15-minute cells may be divided into four 7.5-minute cells.

Intermediate-scale hydrography and transportation DLGs are sold on compact disc-read only memory (CD-ROM). Each disc contains all the 15- by 15-minute cells within the 1:100,000-scale quadrangles that cover a State or States. Fourteen sectional regions in the United States covering the conterminous 48 States and Hawaii are available.

Presently, intermediate-scale DLGs are sold in five categories or units:

  1. Public Land Survey System

  2. Boundaries

  3. Transportation

  4. Hydrography

  5. Hypsography

Large-Scale DLG's

Large-scale DLGs are produced in 7.5-minute units that correspond to USGS 1:20,000-, 1:24,000-, and 1:25,000-scale topographic quadrangle maps. However, some older units in the western United States cover 15-minute areas and correspond to maps at 1:62,500 scale. The unit sizes in Alaska vary depending on latitude. Units south of 59 degrees N., cover 15- by 20-minute areas; between 59 and 62 degrees N., 15- by 22.5-minute areas; between 62 and 68 degrees N., 15- by 35-minute areas (all values are latitude and longitude, respectively).

Large-scale DLGs are available in nine categories or units:

  1. Public Land Survey System, including township, range, and section line information

  2. Boundaries, including State, county, city, and other national and State lands such as forests and parks

  3. Transportation, including roads and trails, railroads, pipelines, and transmission lines

  4. Hydrography, including flowing water, standing water, and wetlands

  5. Hypsography, including contours and supplementary spot elevations

  6. Nonvegetative features, including lava, sand, and gravel

  7. Survey control and markers, including horizontal and vertical positions (third order or better)

  8. Manmade features, including cultural features not collected in other data categories such as buildings

  9. Vegetative surface cover, including woods, scrub, orchards, vineyards, and vegetative features associated with wetlands

Small-Scale DLGs

Small-scale DLG's are derived from the USGS 1:2,000,000-scale sectional maps of the National Atlas of the United States of America. Small-scale DLG's were revised from 1990-95 sources.

Small-scale DLG's are sold in five categories or units:

  1. Boundaries

  2. Transportation, including roads and rails, railroads, pipelines and airports

  3. Hydrography

  4. Manmade features, including built-up areas, capitasl, county seats, populated places, and population range

  5. Public Land Survey System, including land grants, township, range, and subdivisions of the public lands.

Product Delivery Format

The standard distribution product is a CD-ROM. Additional medias are available. The State DLGs at 1:2 million-scale are available on one CD-ROM, and the first of a series of CD-ROMs has been released with 1:100,000 intermediate-scale transportation and hydrography data. The 1:100,000-scale data (all layers, all formats), the State 1:2 million-scale data (SDTS format), and the 1:24,000-scale data (SDTS format) are also available online.

Contact any Earth Science Information Center for additional information or placing an order

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Map Accuracy Standards

The U.S. Geological Survey (USGS) publishes maps and other products at high levels of accuracy. Dependability is vital, for example, to engineers, highway officials, and land-use planners who use USGS topographic maps as basic planning tools.

U. S. Geological Survey - Map Accuracy Standards

Fact Sheet FS-078-96 (September 1997)

Map Accuracy

An inaccurate map is not a reliable map. "X" may mark the spot where the treasure is buried, but unless the seeker can locate "X" in relation to known landmarks, the map is not very useful.

The U.S. Geological Survey (USGS) publishes maps and other products at high levels of accuracy. Dependability is vital, for example, to engineers, highway officials, and land-use planners who use USGS topographic maps as basic planning tools.

As a result, the USGS makes every effort to achieve a high level of accuracy in all of its published products. An important aim of its accuracy control program is to meet the U.S. National Map Accuracy Standards.

National Map Accuracy Standards

To find methods of ensuring the accuracy of both location (the latitude and longitude of a point) and elevation (the altitude above sea level), the American Society for Photogrammetry and Remote Sensing - an organization actively involved in the science of making precise measurements from photographs (photogrammetry) and acquiring information from aerial photographs and satellite image data (remote sensing) - set up a committee in 1937 to draft accuracy specifications. Sparked by this work, agencies of the Federal Government, including the USGS, began their own inquiries and studies of map accuracy standards. In 1941, the U.S. Bureau of the Budget issued the "United States National Map Accuracy Standards," which applied to all Federal agencies that produce maps. The standards were revised several times, and the current version was issued in 1947. (The standards are printed at the end of this fact sheet.)

As applied to the USGS 7.5-minute quadrangle topographic map, the horizontal accuracy standard requires that the positions of 90 percent of all points tested must be accurate within 1/50th of an inch (0.05 centimeters) on the map. At 1:24,000 scale, 1/50th of an inch is 40 feet (12.2 meters). The vertical accuracy standard requires that the elevation of 90 percent of all points tested must be correct within half of the contour interval. On a map with a contour interval of 10 feet, the map must correctly show 90 percent of all points tested within 5 feet(1.5 meters) of the actual elevation.

Factual Errors

There are other kinds of errors in map making. Names and symbols of features and classification of roads or woodlands are among the principal items that are subject to factual error. Mapmakers cannot apply a numerical value to this kind of information; they must rely on local sources for their information. Sometimes the local information is wrong. Sometimes names change or new names and features are added in an area. The USGS cartographers and editors check all maps thoroughly and, as a matter of professional pride, attempt to keep factual errors to a minimum.

"Errors" resulting from selection, generalization, and displacement are necessary results of mapping complex features at reduced scales. In congested areas, large buildings may be plotted to scale and the smaller buildings may have to be omitted; in showing buildings of irregular shape, small wings, bays, and projections usually are disregarded, and the outline is shown in general form. At map scale, it may not be possible to show each of several closely spaced linear features in its correct position. In such cases, one feature, such as a railroad, is positioned in its true location and others, such as parallel roads or rivers, are displaced the minimum amount necessary to make each symbol legible or are omitted to make the highest priority symbol legible.

United States National Map Accuracy Standards

With a view to the utmost economy and expedition in producing maps that fulfill not only the broad needs for standard or principal maps, but also the reasonable particular needs of individual agencies, the Federal Government has defined the following standards of accuracy for published maps:

1. Horizontal accuracy. For maps on publication scales larger than 1:20,000, not more than 10 percent of the points tested shall be in error by more than 1/30 inch, measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50 inch. These limits of accuracy shall apply to positions of well-defined points only. Well-defined points are those that are easily visible or recoverable on the ground, such as the following: monuments or markers, such as bench marks, property boundary monuments; intersections of roads and railroads; corners of large buildings or structures (or center points of small buildings). In general, what is well-defined will also be determined by what is plottable on the scale of the map with-in 1/100 inch. Thus, while the intersection of two roads or property lines meeting at right angles would come within a sensible interpretation, identification of the intersection of such lines meeting at an acute angle would not be practicable within 1/100 inch. Similarly, features not identifiable upon the ground within close limits are not to be considered as test points within the limits quoted, even though their positions may be scaled closely upon the map. This class would cover timber lines and soil boundaries.
 

2. Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error by more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.

3. The accuracy of any map may be tested by comparing the positions of points whose locations or elevations are shown upon it with corresponding positions as determined by surveys of a higher accuracy. Tests shall be made by the producing agency, which shall also determine which of its maps are to be tested, and the extent of such testing.

4. Published maps meeting these accuracy requirements shall note this fact in their legends, as follows: "This map complies with National Map Accuracy Standards."

5. Published maps whose errors exceed those aforestated shall omit from their legends all mention of standard accuracy.

6. When a published map is a considerable enlargement of a map drawing (manuscript) or of a published map, that fact shall be stated in the legend. For example, "This map is an enlargement of a 1:20,000-scale map drawing," or "This map is an enlargement of a 1:24,000-scale published map."

7. To facilitate ready interchange and use of basic information for map construction among all Federal map making agencies, manuscript maps and published maps, wherever economically feasible and consistent with the use to which the map is to be put, shall conform to latitude and longitude boundaries, being 15 minutes of latitude and longitude, or 7.5 minutes, or 3.75 minutes in size.

How To Obtain More Information

For information on these and other USGS products and services, call 1-888-ASK-USGS, or use the EARTHFAX fax-on-demand system, which is available 24 hours a day at 703-648-4888.

Please visit the USGS

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