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Academic Angle

April 1, 2004
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GPS and the GCDB for corner reconnaissance.

Editor's Note: POB is happy to welcome Mickie Warwick, PLS, to our list of columnists. Mickie will be writing on issues related to the educational sector of the profession.

I didn't grow up thinking I wanted to be a surveyor. Not many of us did I don't think. Not unless it was the family business. I did know I wanted to work outside and wanted to do something that would be interesting and challenging, something that wouldn't be the same every day. Surveying certainly fits this description. Now I have been teaching surveying for more than 10 years and have found that I enjoy teaching as much as I do surveying. My students are definitely both interesting and challenging.

The best thing about surveying to me is the amazing variety of topics it involves. In one week alone I can discuss the astronomical readings for determination of latitude and longitude found in the 1804 field notes from the Hunter-Dunbar explorations of the Ouachita River, discuss a legal case between a city and a state agency, speak to a historical society about the original surveys of the Louisiana Purchase Territory, work with data from a static GPS survey, and download maps, aerial photos and NGS datasheets into a GIS to plan for a future GPS survey.

But that variety and the quickly changing technology can be overwhelming, too. It seems that just when I am comfortable with one type of technology or software, it becomes obsolete. But, like I said, it is certainly not boring. And, despite the frustration that sometimes accompanies the integration of new technology, I'm always excited when something new helps me save time and effort while surveying. The Geographic Coordinate Data Base (GCDB) that I describe in this article has done just that-saved me and my students time and effort.

Intro to GCDB

A few years ago, I was asked to be the chair person of the Public Land Survey System (PLSS) I-Team in Arkansas. The group's mission is to coordinate efforts (both governmental and private) to make a PLSS layer for use in a statewide GIS. It was through this group work that I learned of a super database for corner reconnaisance work: the GCDB.

In many of the PLSS states, the GCDB is in use and the coordinates are available through the Land Survey Information System (LSIS) located at www.geocommunicator.gov/lsi/index.html. The coordinates are based on the original GLO field notes, official surveys, local surveys and geodetic control. They conform to the Bureau of Land Management (BLM) rules for the subdivision of townships and sections. All the data is weighted according to its estimated accuracy and a least squares adjustment is made. The GCDB has approximate coordinates on PLSS corners down to the 1/16th section. These were generated for use in a land management GIS and not as a surveying product. The following disclaimer is in bold on the homepage: This data is intended for mapping purposes only and is not a substitute for a legal land survey. The coordinates are not available in all states.

I have found the coordinates in the GCDB to be very useful for corner reconnaissance. While these coordinates are available as a file type that is compatible with many GIS software packages, it is not necessary to use GIS in order to use the coordinates. The common GIS "shapefile" contains several different file types, one being the Database File format (.dbf file), which can be opened in Microsoft Excel. The .dbf file contains information that identifies the USPLSS corner and gives its approximate latitude and longitude. These may be entered into a recreational or other GPS receiver as waypoints and used for field reconnaissance.

From the LSIS homepage, users select "Download Data." This brings up an interactive map where the area of interest can be selected by county, township, state, etc. Users download the desired townships as "BLM (GIS Shapefile format)." There will be several files. The "coords.dbf" file will look like that seen in Figure 1 when opened in Microsoft Excel.

The "lndkey" column gives the state, principal meridian, township and range information. There are extra zero digits. In this example, the top line is Arkansas, Fifth Principal Meridian, Township 14 North, Range 27 West. There is a key to the numbers assigned to the different principal meridians on the LSIS webpage; it can be located under the T/R (Township/Range) tab of the map page.

The "pntid," or point ID, column identifies the corner within the township. The first three digits symbolize the x coordinate and the second three digits symbolize the y coordinate. The first three digits will be 100 for the entire west range line of the township, 140 will be nominally 40 chains to the east, 160 will be 60 chains to the east. Instead of 180, the system jumps to 200 for the section lines along the east side of the westernmost column of sections in the township. The same system applies for the tiers, with 100 being the south line of the township. So, 200700 is the north section corner between sections 5 and 6.

The latitude and longitude in decimal degrees are provided and may be entered manually into any GPS receiver that will save waypoints. These coordinates are given in NAD 27 though, and many GPS receivers will default to WGS84. If a GPS receiver doesn't have an option for NAD 27, a free program can be downloaded to convert the coordinates to NAD83. Called NADCON, this program is available from NGS at www.ngs.noaa.gov/PC_PROD/pc_prod.shtml. While NAD83 is not identical to WGS84, it is close enough for this application. If a mapping grade GPS receiver is used, the coordinates can be imported into the receiver after some edits to the table.

GCDB in Action

Several of my students at the University of Arkansas-Monticello and I used coordinates from the GCDB to look for existing monuments in the Ozark and Ouachita National Forests in Arkansas. These forests are quite rugged and it would have taken a good deal of skill and time to locate many of the monuments without the coordinates. We found monuments that had not been maintained since 1932, so there were no metal tags or painted trees or other easily visible evidence. If we got on a road that was not shown on our map, or was in a different place than the map showed, we knew immediately that we were not where we thought we were. A lot of time was saved by using the coordinates.



For the class project, only existing BLM monuments were used, but the coordinates would be extremely useful for other corner searches. It is a quick method to get to an approximate search area. It is wise to check the approximate error between the coordinates and a found corner in the vicinity and apply an "index correction." The GPS receiver allows users to go around obstacles and then return to the approximate true line. Topographic calls can be checked and the current locations of creek crossings, etc. can be saved for future analysis.

Coordinates scaled from USGS quad maps can be used in a similar way. We found every NGS control monument we looked for by using the published coordinates. One was seven-tenths of a mile up a very steep ridge. It was missing the brass cap, and only the stem was still there, cemented into the bedrock.

Using the GCDB coordinates to search for monuments saved me and my students a significant amount of time. While the coordinates are not conclusive in any way, they are a valuable piece of evidence to take to the field to begin a corner search. I am sure we will hear more on how the GCDB has helped other surveyors in their work.

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