I recently submitted some coordinates I acquired via ArcGIS to the BLM to go out into the field and pinpoint some locations.

When I did the work at the computer I used ArcGIS and leveraged the BLM's CadNSDI dataset to pinpoint PLSS section lines. When I heard back from the BLM, they claimed that my points were not accurate to footage calls in the field.

I started doing some digging and realized that there are two standards utilized by the BLM for PLSS data. These two datasets are the CadNSDI(which I used) and the GCDB, which based on the information at the following link, seems to be an older standard:


Why do these two datasets not line up, and which one is more accurate for comparing footage calls from section lines in the field?

Both of the datasets are in NAD83 but do not line up with errors in the range of 20 feet(I am projecting into a State Plane projection). I can't seem to wrap my head around what is happening here. From my perspective it seems like the BLM serves out two datasets that don't line up and then when you submit coordinates to them they say they are inaccurate when using the newer standard.


The latest data is generally the most accurate, but you have to realize that some of those corner locations were originally established using a wagon wheel to measure distance, and a Gurley Transit, or Compass to establish line. Maybe a sunshot was taken along the way. 20 feet of error between original corners is nothing. Reading some of the survey notes, you may find out that an entire township (36 square miles) was surveyed, and monumented with scribed stones in 19 days.

Many of the original corners have not been revisited often or possibly at all. (If your state has monument records, you can check those to see if anyone has visited the corners of interest.)

The PLSS data is provided in NAD83 Geographic positions, and you should reproject it to something local to your area (Which you have) such NAD83 US Feet in Colorado North Zone. This will provide you with better distance comparisons. Then it is necessary to reduce the distances between the points from a grid distance to a ground distance. This involves taking the grid distance and multiplying it by the inverse of the combined factor (Scale Factor * Sea Level Reduction Factor).

The corner locations provided are basically a best guess solution based upon the surveying notes taken during the original survey, unless the corner has been replaced or upgraded, then the location should be somewhat more accurate.

I have seen 450 feet of positional error in GCDB coordinates versus real world locations on more than one occasion.

In other words, the locations of the corners provided by the GCDB are only a guide to assist you in locating the corner, the real locations can only be derived by actually occupying the corners.

This subject also gets much deeper than my explanation above.

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