Projected geographic coordinates have several advantages for the land surveyor. Geographic points and linework drop directly onto aerial imagery and inexpensive handheld GPS receivers can be used for search.
However many small and medium size projects use local coordinates, ground distances and true or record bearings. Local coordinates are familiar, and mapping is often recorded with a true or record basis of bearings.
The ArcGIS Pro tools in this project provide a link between the local project coordinates and projected geographic data in a GIS project.
The tools use the geoprocessing environment variables workspace and outputCoordinateSystem.
The tools also assume points are saved in a Feature Dataset called Points and use a
Feature Class template called Points\PointsTemplate. Initialize the points templare
from the ArcGIS Pro Python window -
sys.path.insert(0, r'D:\Projects\Python\ArcGIS_Coord_Transform')
from tools.utils import create_points_template
create_points_template()
Start by creating project boundaries and other features using traditional CAD tools. Create points and export a PNEZD points file. Import these local points into an ArcGIS geodatabase with the Import PNEZD tool using no transform.
Next, set up an ArcGIS Pro project using an appropriate projected coordinate system. Create a feature class with one or more projected geographic points in the ArcGIS project. These points represent the target positions for corresponding local points. Initially this might be a rough location picked from an aerial image or geographic coordinates from a map or GPS shot. A single point and rotation will be enough to get started.
Now use the Calculate Transform tool to generate the four-parameter transform file used to import points and linework into the ArcGIS project. Input for the Calculate Transform tool can be entered manually or placed into a Transform.xml parameter file. In many cases transform parameters will be recalculated as more accurate geographic positions become available.
Finally, use the Import CAD and Import PNEZD tools with the transform parameters to create feature classes and transform coordinates to the projected coordinate system. If in the future the transform in updated features can either be reimported using the new transform parameters or run through the Transform Features tool, once using the old transform parameters in the Inverse direction and again using the new parameters in the Forward direction.
The parameter file has the four parameters for a similarity transform: x/y translation, rotation and scale. They are expressed as a0, b0, a1, b1 where -
a1 = scale * cos(rotate)
b1 = scale * sin(rotate)
a0 = x1 - a1 * x0 + b1 * y0
b0 = y1 - b1 * x0 - a1 * y0