custom map projections

Discussion created by aimong on Apr 10, 2012
I have a workflow similar to this and was wondering if it is possible to do it with code (particularily calculating the northing and easting offsets and adding it to the values in the custom map projection). just wanted to get opinions before attempting.

1. Create two sets of corresponding point shapefiles: one in local grid, the other in UTM.
2. Make sure the UTM data has its coordinate system defined. Do not define a coordinate system on the grid data.
3. Add both to ArcMap.
4. Open data frame properties and select the coordinate system tab.
5. Create a new, custom projected coordinate system (pcs).
a. Name: anything you want
b. Geographic coordinate system: same as the UTM zone, likely NAD 1983
c. Projection: Local
d. Parameters:
i. Longitude of center/latitude of center: enter if known, if not, try to identify a point near the center of the data or near nice ???even??? grid coordinates
ii. Scale factor: leave = 1 for now
iii. Azimuth: Try entering the value you have but it???s almost certainly incorrect (that???s because angle is based upon the UTM zone which already has an angle from north)
iv. False easting/northing: if you know the center coordinate, use those values
e. Units: whatever the grid is in
6. OK all dialogs. You will see the UTM data being projected on-the-fly to the new custom pcs. The ???unknown??? data is just displayed???because ArcMap can???t reproject it.
7. You can now modify/edit the data frame???s coordinate system. Change the northing and eastings by calculating distance from control point, etc. You want to line up the state plane data with the grid data.