An old point layer created from PLSS grid does not line up with current PLSS grid layer

253
1
01-31-2019 08:24 AM
OleksandrStefankiv
New Contributor

I have two vector layers which I am having trouble lining up. First, I have a point layer that was developed a decade ago to sample information from Public Land Survey System records in Washington and Oregon State. Second layer I have is PLSS grid polygon layer recently downloaded from Bureau of Land Management. The points are not just nodes, but also were placed at various distances along the grid lines.

Spatial reference:

  • Point layer is projected in NAD83 / BC Albers, EPSG: 3005.
  • PLSS grid polygon layer is not projected, instead it has GCS North American 1983, EPSG: 4269

I have tried to project the grid layer to EPSG: 3005 and several others. Similarly, I have tried to reproject the point layer. Essentially, sampled points should line up with PLSS grid corners and other point placed along the grid lines, however all of my attempts to project each layer have yielded no desirable results, points are always off. Unfortunately, no documentation is available on development of the point layer, therefore I do not know whether previous PLSS layer used a different projection, but I would expect BLM PLSS layers to be the same to layer available a decade ago.

 

Please help to identify possible solutions, as the point layer contains a lot of valuable information which would take very long to replace or correct manually.

0 Kudos
1 Reply
BenKnott
Esri Contributor

Hi Oleksandr, I'm guessing that you've moved on by know, but in case there's still interest. I'll start by saying I'm not a licensed Land Surveyor, but I've used the heck out of this data.

Background: The data your working with is a GIS representation of the Public Land Survey System. In Oregon and Washington, the PLSS Monument (points) were originally established by Land Surveyors in the 1850's. In the 1980's and 90's, a sampling of the coordinates representing PLSS Monuments were digitized from various sources of information, like USGS Topo Maps, and brought into a spatial database. The estimated accuracy of these digital representations of PLSS Monument locations was/is typically estimated at +/- 40'. Meaning, if you drove to that exact coordinate, then you *should* be able to find the original monument within a buffer area of 40'. Now with the consumer introduction of GPS, more and more PLSS Monument Locations are being captured at a much tighter accuracy (depending on the equipment and other variables). As the location accuracy of PLSS Monuments improves, the GIS representation of the PLSS boundaries and surrounding monuments improves. To date, only a small percentage of the total number of original PLSS Monuments have been revisited with GPS since they were originally established.

Response to Question: From the image you provided, it looks like the GIS point layer didn't account for offsets in the original PLSS. Original surveyors did this because you can't lay out a grid of perfect squares over a larger distance on a round earth. So the PLSS makes corrections at a certain number of townships. That's most likely what your seeing along the southern two Townships.

As for Solutions: you could look at the ArcGIS Pro Conflation tools and they should help with your non-offsetting points. Where you might run into trouble is where you have a single point within proximity of two offsetting points (image above). 

0 Kudos