How to Georeference Survey Data
This is a two part question.
I have survey data collected with a Total Station that I need to georeference to GPS coordinates. The GPS points were only taken at the 4 corners of the plot, and hundreds of points were collected by the Total Station within the plot. The arbitrary coordinate system used with the Total Station is XYZ=1000, 1000, 100. I'm using ArcMap-ArcInfo 9.3.1 and I do not have the Survey Analyst extension. I have used the Georeferencing toolbar to georeference rasters. What is the best way to get all of the points into real coordinate space based only on the 4 GPS coordinates? Is there more than one way to do this without distorting the relative positions of the points?
The data is in an Excel file; in ArcCatalog, I right-click it to create feature class from XY table. Is this the best way to import the data into ArcMap or do I need to create a coverage or something else?
Mapping Center Answer:
This easy to do but will require a few steps and the use of the Spatial Adjustment methods to transform the data. This can all be done in 9.3.1 without extensions. Here are the steps...
Get the data into a shapefile
There might be a GP tool to get Excel data directly into a shapefile but I can’t think of any offhand, so the manual way is to "create the feature class from XY", as you have already done, then export the "layer" to create a proper shapefile.
Create a text file with the GPS points
Create a text file and enter the four known GPS points as long/lat coordinates separated by a tab. Each point goes on a new line. The coordinates should be in decimal degrees. It should look something like:
This will be your control points file in the next step.
Transform the data via spatial adjustment
The idea is that you will create links between known locations in the data and the corresponding GPS points in the text file.
- In ArcMap, start editing and display the Spatial Adjustment toolbar.
- Zoom to one of the known corners of the data that matches a GPS coordinate.
- On the Spatial Adjustment toolbar, click the Spatial Adjustment menu-> Links -> Open Control Points to open the control point file, and select the text file containing the GPS points created above. This will display the control points window, hopefully containing the four points.
- Right click the correct GPS text entry in the window matching the approximate location you have zoomed into and select Add Link. This will create half a link with "to" end at the GPS coordinate. You probably won't be able to see it on the map.
- Now you need to interactively place the "from" end of the link at the corresponding location in the data. Just mouse to the location and click once.
- Repeat this for the other three points.
Transform the data
- On the Spatial Adjustment toolbar, click the Spatial Adjustment menu -> Set Adjust Data.
- Confirm or set what data you want to adjust. This should be the entire layer you imported.
- Next, set the transformation type by clicking the Spatial Adjustment Menu -> Adjustment Methods and select Transform – Affine. This is the default.
- Finally, perform the adjust by clicking the Spatial Adjustment menu -> Adjust.
If the transformation doesn’t look right, you can undo the operation then work out what went wrong. Usually this is an incorrect GPS entry or the "from" ends are wrong, etc. You should probably also define a spatial reference for the shapefile when you’re finished -- use the Define Projection tool to do this.
I hope this all makes sense. Let us know if there are any questions!
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