topic Centroid definition and coordinates system in Coordinate Reference Systems Questions

Centroid definition and coordinates system

PabloOrdoñez1 — Fri, 21 Oct 2016 02:38:27 GMT

I'm calculating the centroids for a polygon (the city of Morelia in Mexico), but the coordinates that I'm getting are not inside the polygon and are way off from where the polygon is. What could be happening? Are the coordinates from the centroid given in a different forma? (The ones I'm getting are x= 2575360 and y=852065.8)

I used Stata to double check and I'm getting the same result.

Thanks!

Re: Centroid definition and coordinates system

DanPatterson_Retired — Fri, 21 Oct 2016 02:48:02 GMT

A centroid need not be within a polygon, Consider a C or U shaped object we just expect it to be, but mathematically that need not be the case.

Centroid - Wikipedia

If you have an advanced licence you can calculate 'centroid' features one the standard and the other that does some fiddling (label point) that makes the centerish point inside

Feature To Point—Help | ArcGIS for Desktop

Re: Centroid definition and coordinates system

PabloOrdoñez1 — Fri, 21 Oct 2016 15:52:28 GMT

Thanks Dan, I'll check what kind of license we have and try and use the feature to point. However, I still don't understand how the centroid of a closed polygon (a city in this case) could lie outside of it and seem to be so far off. My guess is that the coordinates I'm getting are in some other format and that that's why I'm getting such an odd result.

Re: Centroid definition and coordinates system

DanPatterson_Retired — Fri, 21 Oct 2016 16:54:30 GMT

well the other way... consider two perfect similar circles on opposite sides of a river. The two parts could form a multipart polygon, called your city. Multipart features consist of seemingly separate geometry where in actual fact they form a whole and occupy only one record in the table. I would venture that the centroid is now wet since it would reside in the middle of the river

Re: Centroid definition and coordinates system

DarrenWiens2 — Fri, 21 Oct 2016 17:13:52 GMT

Add two new fields to your polygon file and calculate the centroid coordinates, using calculate geometry. You can control what CRS the coordinates use.