# Finding lowest elevation in multiple polygons using DEM? [closed]

I have a polygon layer with six polygons. I need to have the lowest elevation point in each polygon using a DEM. The following explanation https://support.esri.com/en/technical-article/000011761 (Zonal Statistics, raster calc, raster to point) really helped me.

I have managed to do it, but the problem is that the shapefile with the lowest point for each polygon does not match with the original polygons. I mean, lowest point A belongs to polygon B and lowest point F belongs to polygon C, ... I cannot manage to have a unique identifier so that I can see that the lowest point of polygon A is point A. I have tried to make a new ID for each polygon (1 till 6) and I have tried the original ID's (which were non-consecutive, such as 1,3,4,12) but then I only get the lowest point of all polygons, so only one point remains ...

Someone who knows how to fix this?

• Use modelbuilder to iterate over the polygons individually. – Hornbydd Jun 28 '19 at 13:09
• Do you just need to know what the minimum elevation is within each polygon, or also where in the polygon that lowest elevation is located? – ycartwhelen Jun 28 '19 at 13:16
• I would take another look at that article. How I'm understanding it, it should give you the highest (or lowest) point for every single polygon. I will admit the wording is a little confusing in places. – ycartwhelen Jun 28 '19 at 13:31
• I am looking for the minimum elevation in each polygon and also where that is located. Especially the location is important. – Lies Jun 28 '19 at 13:45
• Are you using the Raster Calculator from ArcGIS Spatial Analyst or something else? – PolyGeo Jun 29 '19 at 4:35

## 1 Answer

The link you included in your question should do exactly what you're looking for (once max is swapped for min). The first part will make a raster layer with the min value within each polygon. The second part will compare this min layer with the original DEM to find all the pixels where they are the same (it should be one per polygon), and then set everything else to NoData. The third step will convert these remaining points (one per polygon) into points.

To match low points with their original polygons do a spatial join like is described in this question.

• I have managed to do it, but the problem is that the shapefile with the lowest point for each polygon does not match with the original polygons. I mean, lowest point A belongs to polygon B and lowest point F belongs to polygon C, ... I cannot manage to have a unique identifier so that I can see that the lowest point of polygon A is point A. I have tried to make a new ID for each polygon (1 till 6) and I have tried the original ID's (which were non-consecutive, such as 1,3,4,12) but then I only get the lowest point of all polygons, so only one point remains ... You know how to fix this? – Lies Jul 1 '19 at 12:07
• Try doing a spatial join between the polygons and the output point file, like is described here: gis.stackexchange.com/questions/292797/… – ycartwhelen Jul 1 '19 at 12:13
• Ok it worked! Many thanks! – Lies Jul 1 '19 at 13:04