Graphical Locater Home Page

Key Words: Graphical Locator, point-and-click position data, USA maps, state maps, HUC region maps, custom maps, township-range-section (TRS) conversion, UTM conversion, degrees-minutes-seconds (DMS) conversion, HUC maps and information.

Quick Instructions

Introduction

News

Usage Statistics (calendar year 2006)

• About 16,804 of the total 31,277 static files were accessed 1 or more times
• The static file access rate is 734,167 requests per year.
• The custom image generation rate (zoom, ppih, czoom and cust) is about 40,790 requests per year.
• Main program usage (virtual pages) is 382,946 requests per year.
  • Graphical interface 22.9%
  • TRS-data interface 49.7%
  • XY-info interface 5.6%
  • XY-data interface 21.8%
• Total access rate is about 1,157,903 requests per year or 3172 requests per day.

Area of Coverage

Graphical Locater Starting Maps

Graphical Locater Level 2 Maps

• Standard 1:100,000 Maps: These maps cover 1 degree of longitude by 0.5 degree of latitude, exactly the same as the USGS or BLM map series. The title of each map is the official USGS map name, except for Puerto Rico and the Virgin Islands, where Mike Ivie and I made up the names. These maps are all 950 by 620 pixels, which results in a square pixel near the center of the USA (North-South). The panning table at the top of the page allows moving to the next map in any direction. Directions are unlinked when the corresponding map is not available. All areas with 1-degree DEM coverage are included. The line and name data cover only the USA. The limits of the map are at the bottom of the page.

• Conterminous USA HUC Regions: These maps, as well as maps of the subregions, accounting units and cataloging units are also available by following the HUC link on any Graphical Locater result page. Accounting unit maps are not available when they duplicate the subregion. See our HUC Home Page for more information.

• The 50 American States: Each state has 4 map versions.

• The Conterminous USA: There are currently 9 versions available.

• Custom Level 2 Maps: These maps support specific projects. They must be pixel-centered maps of straight latitude and longitude, but size, content and colors may be anything. The spacing of the lines of latitude and longitude must both be linear, but need not be equal. As of September 7, 2003 the name of the referring html filename is no longer important. Formerly, the referring html file name must have started with xyXYnm, where xy is the southwest corner, XY is the northeast corner, n is the number of x pixels and m is the number of y pixels. The format is (F7.3,F6.3,F7.3,F6.3,2I4) with "0" (zero) padding for any spaces. For example, 116.02840.916115.37741.63908391234.html is the HUC map for the North Fork Humboldt River. After September 7, these data with an additional coordinate pair are included as a packed argument to the gl program. This is to avoid using the HTTP_REFERER variable which is not reliable for some Internet service providers. The new data are xyXYnmUV, where UV gives the location of a marked point, or 0 for none. The format is (F7.3,F6.3,F7.3,F6.3,2I4,F8.4,F7.4) with "0" (zero) padding for any spaces. For example: "http://www.esg.montana.edu/cgi-bin/gl?112.00045.500111.00046.00009500620000.000000.0000" gives the link for the gl program to use the Bozeman 1:100,000 map with no marked location. That should be all you need to make your own custom maps or you can use Graphical Locater Custom Mapper to make them for you.

• Zoom Maps: These are just custom maps that are made on demand by a Graphical Locater support program. The zoomed point location is shown in the center of the map, in yellow.

Returned Information Details

1. Latitude/Longitude: The location data in decimal degrees of arc with the degree symbol and hemisphere (North and West). Four decimal digits translates to less than 10 meters. The output is also given as degrees (°), minutes(') and seconds ("), the last has 1 decimal digit. Decimal minutes is the seconds divided by 60 added to the minutes.

2. The legal description: This includes, in order, the meridian, township, range, section and state. This program has this information only for the western states. Note that township, range and section define an unique square mile only when the meridian is also given.

3. UTM: This is the Universal Transverse Mercator zone and x,y coordinates in meters. This conversion is done using the Geodetic Reference System of 1980 (GRS 1980) of the North American Datum of 1983 (NAD 83). Many projects are still using the Clarke Spheroid of 1866 of the NAD 27. Note that a UTM coordinate is meaningless unless the zone is also reported. The UTM coordinates can be used to calculate the distance between to points, as long as the zone is the same.

4. The pixel size: This is given only for entry from a clickable map. The first value is for the E-W direction and the second for the N-S direction to the nearest meter. This is a measure of the expected accuracy. Distances are based on the GRS80 ellipsoid. Non-square pixels are the result of map distortion in the North-South direction. Map distortion does not affect location accuracy, except for the change in pixel size.

5. The distance from the marked point: This is given only for custom maps which already have 1 point defined. Distances are based on the GRS80 ellipsoid and reported to the nearest meter.

6. The elevation: The height above mean sea level in meters and feet. This is based on simple bilinear interpolation of the original 1-degree (3 arc second) digital elevation model (DEM) data. The median absolute difference between Graphical Locater elevations and the elevation reported for over 30,000 named places in the us_concise file of the GNIS was 5.5 meters. Of the 10 largest errors, all were in the name file. As with all grid interpolations, the elevation of peaks will be low and the elevations of dips will be high. The local roughness gives some measure of the point elevation uncertainty.

7. The gradient: This is the maximum slope through the point. Slope is feature of a line, not a point. The gradient is calculated from the X-direction slope and Y-direction slope. The directional slopes are based on bilinear interpolation of elevation over a radius of 35 meters. The reported gradient is thus a very local measure (about 1 pixel on the standard maps). Any location within 1 arc second of a whole degree cell edge is moved slightly inward to prevent the need to bridge across the DEM units and the discrepancies that occur there.

8. The aspect direction: This is the direction of downhill gradient. I am using 0 for North, 90 for East, 180 for South and 270 for West. The standard abbreviation of the 8 compass directions is given after the number value. Note that this is a circular statistic with 359 and 1 being almost the same, true North. The aspect calculation is based on the X-direction and Y-direction slopes. The directional slopes are based on bilinear interpolation of elevation over a radius of 35 meters. The reported aspect is thus a very local measure. Any location within 1 arc second of a whole degree cell edge is moved slightly inward to prevent the need to bridge across the DEM units and the discrepancies that occur there.

9. The local roughness: This is simply the standard deviation of surrounding elevation values. This calculation uses the nearest 16 elevation values in the original 1-degree (3 arc second) digital elevation model (DEM) data. This corresponds to a radius of about 125 meters. Multiple 1-degree units are never combined for this calculation to avoid the discrepancies that occur there. Variation in elevation is only 1 of 3 components of terrain roughness. Variation in gradient and variation in aspect are too demanding to calculate here. The local roughness interpretation is based on almost 100,000 equally spaced points across the USA. Flat is for no variation in elevation; slight is above zero and below the 25th percentile, average is between the 25th and 75th percentiles, moderate is between the 75th and 95th percentiles and extreme is over the 95th percentile.

10. The location (X, Y; Z): This is my choice for standard data entry, many others are possible. West longitudes are negative and X (longitude) comes before Y (latitude).

11. The state, county and FIPS code: This determination is based on a 850 m raster grid created from 1:2,000,000 DLG data. It will be a little "rough on the edges". A test run against the GNIS gave the error rate at 3.16%, but some of the errors are in the GNIS. Many of the errors are named peaks that occur on the county line.

12. The hydrologic unit name and code: These are the USGS 8-digit HUC composed of 4 2-digit codes that give the region, subregion, accounting unit and catalog unit. The "Place point in HUC" link allows you to mark the current position in the current HUC map. The HUC determination is based on a 850 m raster grid created from the 1:2,000,000 DLG data that were further refined in our lab. Our refined version should match the USGS intent, except for California subregion 2 accounting unit 1 and subregions 3 and 4. In these areas, some units were constructed so poorly that they have no utility for our purposes and they were restructured to better match drainage borders. The HUC is linked to our own HUC summary page that features 3 graphical locater maps for each unit. Because of the grid size, the HUC determination will be a little "rough at the edges". A test run of almost 1000 sample sites in one of my projects, gave the error rate at 0.85 %. It also discovered one error in my data! Most errors occur at the edges of the units (mouth or headwaters) and these can be easily double checked by selecting a more central location.

13. The Omernik ecoregion name and number: These are the 76 regions used by Omernik, 1987 for the conterminous United States. The more or less typical flag is also given. The ecoregion determination is based on a 850 m grid and will be a little "rough at the edges". The error rate should be similar to that of the HUC and FIPS determination, but I have no independent data to check.

14. The 1:100,000 map: This is a link to the Graphical Locater 1:100,000 map without checking to see if that map exists.

15. Zoom on that location: This is a link to the Graphical Locater map making program at will make a large-scale (small area) map centered on the selected location. If there are no data within that area, you will get a blank map.

16. Switch to TerraServer: This is a link to Microsoft's TerraServer centered on the selected location.

17. Nearby named places: These are from the Geographic Names Information System (GNIS). Linear features are split into "mouth" and "head" points. The state and county names replace the original FIPS codes, except for "head" locations. Here, the state and county might be wrong, so they are left out. I converted the position data (X,Y as decimal degrees of arc) from NAD 27 to NAD 83 to be more comparable with the TIGER/LINE 1995 line data. The elevation, if given, is the original GNIS data in feet. The distance from the selected location is based on the GRS80 ellipsoid. Selecting the position of a named place makes it the current Graphical Locater position and it allows zooming on that position exactly (or within a few meters due to rounding).

18. The 7.5 minute series topographic maps for that area: This is based on the map names part of the Geographic Names Information System (GNIS). The map names are in a 3 by 3 table similar the "adjoining map table" on many printed maps. The top is North and right is East. The selected point is on the map that is named within the middle cell.

Accuracy

Base Data

• Geographic Names Information System This is described also by the GLIS GNIS page. I obtained a copy of all units, except Antarctica in September 1997. I discarded all records without location data as well as records for springs and wells that lacked real names. I split linear features into "mouth" and "head" records. "Head" records lack state and county. Some linear feature mouths have the state and county of the head location!

• Small-scale (1-degree) Digital Elevation Models. I obtained a complete set of 957 files from the USGS FTP site in September 1997. I added the 420 files for Alaska on March 20, 2000. These are used to generate the shaded relief in all level 2 Graphical Locater maps. They are used by the Graphical Locater program to interpolate the elevation and related measures for any location.

• TIGER/Line '95 I obtained a complete copy of these files from the MSU Library on 6 CD-ROMs. These data are based on the USGS 1:100,000-scale Digital Line Graph Data, but are more convenient for big projects. These are used to map the roads, water-bodies, streams and canals on all large-scale Graphical Locater maps.

• USGS 1:2,000,000-scale Digital Line Graph Data. I have the 1995 USA state files in optional format on CD-ROM. These are used to map the USA roads, water-bodies, streams and canals on small-scale Graphical Locater maps.

• World Data Bank II (WDBII). I have a copy of the original CIA data base from an old CD-ROM. These are 1:3,000,000 scale data that I use for some small-scale maps and for all areas outside the USA.

• Global 30 Arc Second Elevation Data Set (GTOPO30). I have the data for 10 of the 33 tiles. This is a very nice DEM with the grid spaced 30-arc seconds or about 1 km. I use it to generate shaded relief for small-scale maps, including small-scale custom maps.

Base Programs

1. GL - handles all level 2 clickable maps.
2. XY-data - handles form inputs of longitude & latitude and includes forms for degrees minutes and seconds and UTM coordinates.
3. TRS-data - handles form inputs of township, range and section.
4. XY-info - provides an full result page for a known longitude and latitude.

• General Cartographic Transformation Package (GCTP) This is a USGS map projection software library of great value. It permits the transformation of coordinate pairs from one map projection to another. I have the FORTRAN version 2.0.0 of 06/18/92, it is now maintained in c (GCTPc).

• TRS2LL This very useful program by Martin Wefald handles all conversions involving township, range and section. See TRS-data for more information.

• NADCON transforms latitude and longitude coordinate values between the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). I have the official distribution version 2.10 in FORTRAN. This code was used where needed to transform all data to NAD 83.

• NETPBM This is a very useful and widely used set of image conversion and processing programs. Specifically, I use the ppmtogif utility as I never quite learned how to write a gif file with FORTRAN. This program is used in a pipe so that the ppm file is never written to disk.

Acknowledgments