In GIS’, there are tools for finding the coordinates of polylines and polygons. This can be things like the min, max, line start, line mid-point and all sort of variations. A commonly used tool is the centroid. But if you look at the documentation of (for example) the ArcGIS tool for determining centroid geometries, you’ll also see the ‘central’ option. How does this differ? Below we’ll give a quick, visual overview of how these options vary.
We’ve loaded in the country of Japan into our ArcGIS Pro session.
The country of Japan in ArcGIS Pro.
We selected the feature and then ran the Calculate Geometry Attributes tool specifying it to return the centroid x, centroid y AND the central x and central y coordinates.
Specifying the centroid x coordinate in the Calculate Geometry Attributes geoprocessing tool.
We ran the tool which created new attributes with each of the coordinates we specified.
Then we exported it to table, and then ran the XY table to point tool. We ran this twice calling one output “Japancentral” and the other “Japancentroid”.
The two layers produced from the XY table to point tool.
When overlaid on the map, we can see that the green point (centroid) occurs in the ocean, while the red point (central) occurs within the Japan polygon.
The centroid and central points displayed alongside the polygon that they represent.
This is because centroid record the centroid of the polygon irrespective of where that occurs. In this case, it occurs in the ocean. This can be misleading if you need to represent a polygon by a point, representing it as the centroid may display it incorrectly.
Imagine the extreme scenario where you had a C shaped polygon (where the interior of the C was hollow). The centroid would occur in the middle where the C polygon does not actually occur. Imagine the C shape was a vegetation community. The centroid would not actually occur within the community.
The central coordinate on the other hand, will only occur inside the polygon feature. If the centroid is also inside the feature, the central and centroid coordinates will be the same thing. But in our C shaped polygon example, the central coordinate will be in the middle of the C.
Beware when using the Calculate Geometry Attributes tool! Ensure you know what output to expect.