Projection for local survey transit data
I frequently make large scale maps based on survey data which is only tied to a local datum. The data extent is typically less than 1000 m. Is there an appropriate local projection for these data embedded within ArcMap, or should I create one? While I can leave these data as is and project on the fly, I would rather have a standardized spatial reference.
Mapping Center Answer:
If you have at least one point in your data that is also referenced to a lat/lon coordinate then you can use a local projection. OK -- what this means is that you can use that one point in a projection definition that relates DIRECTLY to your local area. That point is called the "tie point".
To do this, for your data frame, for example, right click, select Properties, click on the Coordinate System tab, and then select the following:
Predefined, Projected Coordinate System, UTM, then pick a coordinate system that has the same datum - probably NAD 83. You could use your local UTM zone (you could also do this in State Plane).
Before you click OK, click Modify at the right, then for the Projection, click the drop down arrow, and find "Local". Select this one.
Now set the Latitude of Center and Longitude of Center to be the lat/lon coordinate of your tie point. If this point is not 0,0 in your local system, then input the values as the false easting and false northing which will then align your local datum to the center of the local projection you are defining.
One other parameter you may need to set is the azimuth. If north in your local datum is not true north (i.e., geodetic north), then input the azimuth value from your surveyor.
The only other thing to to worry about is the scale factor. If you know there is a grid-to-ground correction, use the scale factor to take care of this.
If you try this and can't get it to work for you, post your results to the Projections/Projection Engine forum and they can help your further.
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