[GIS] Specifying output coordinate system in ArcPy

arcpybuffercoordinate system

I'm updating somebody else's old Python scripts, and trying to make sure I understand their logic and desired outputs while streamlining the code. In this particular script, it specifies an output projection in the processing environment settings.

import arcpy

Output_CS = "GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',...(etc, etc, etc)"
arcpy.env.outputCoordinateSystem = Output_CS

I think this means that all the subsequent geoprocessing operations (e.g., arcpy.Buffer_analysis) output in WGS84 without needing an additional arcpy.Project_management step.

If that is the case, is there any accuracy risk to running a buffer analysis directly to a geographic coordinate system? In other words, does it do the projecting after the buffer's been appropriately sized?

(The original input data is in a projected coordinate system, one of the NAD83 state plane systems.)

Best Answer

if a feature class has a geographic coordinate system AND the buffer is performed with a linear unit (e.g arcpy.Buffer_analysis(input, output, "100 Feet"), ArcGIS will automatically use geodesic distance, which is great. Just make sure that you specify the unit

Related Solutions

[GIS] Setting custom spatial reference/coordinate system to shapefile using ArcPy

There's a Python code example for doing this under Define Projection.

ArcGIS Desktop – Describing a Coordinate System in Arcpy

Two possible solutions I've found in playing around with the SpatialReference class:

In your script tool parameter's dialog, change the data type of the Coordinate System parameter to Spatial Reference. A spatial reference contains a bit more information than a coordinate system and is what is expected by GetParameter if you're expecting it to create a SpatialReference object.
- Note: In my testing this only works when run as a script tool, not as a standalone Python script, presumably because GetParameter requires the script tool parameter definitions to infer what type of object to create from the input string.
In addition to the above change, you can use the SpatialReference.loadFromString() method to create a spatial reference object from its string representation explicitly. This should work both within a script tool and a standalone Python script. E.g.:
```
import arcpy
input1 = arcpy.GetParameterAsText(0)
input2 = arcpy.GetParameterAsText(1)
srtxt = arcpy.GetParameterAsText(2)
sr = arcpy.SpatialReference()
sr.loadFromString(srtxt)
if not sr.type == "Projected":
    raise RuntimeError("Input coordinate system is not projected.")
```
You'll note I also changed the coordinate system type checking to be more explicit and changed the error handling logic to raise an error which is a bit more Pythonic, though you are free to handle it how you want.

Best Answer

Related Solutions

[GIS] Setting custom spatial reference/coordinate system to shapefile using ArcPy

ArcGIS Desktop – Describing a Coordinate System in Arcpy

Related Question