topic Re: Isohypses - filtering out very small areas of elevation? in ArcGIS Mapping and Charting Questions

Isohypses - filtering out very small areas of elevation?

Michèlevon_Kocemba — Mon, 11 Dec 2017 08:18:09 GMT

Hi!
I am currently creating a topographic map based on DEM data. I created some rasters and merged them with the goal to create some isohypses using the contour tour. Now I used the tool and also the smoothing tool but I got a lot of very small points of elevation in my map that I would like to filter out of my map (I only want to have a minimalistic map showing off the most important features).

Is there any way to do it? I am using ArcGis 10.1. Thank you very much.

Greetings, Michèle.

Re: Isohypses - filtering out very small areas of elevation?

DanPatterson_Retired — Mon, 11 Dec 2017 09:26:17 GMT

you could try

calculat the polyline lengths (isohyets) in the table,
select by some 'appropriate' cutoff length (via inspection, on the sorted list of values),
delete those... below the threshold

Re: Isohypses - filtering out very small areas of elevation?

Michèlevon_Kocemba — Mon, 11 Dec 2017 09:52:14 GMT

Ah, thank you, that seems like a practical solution!

Re: Isohypses - filtering out very small areas of elevation?

DanPatterson_Retired — Mon, 11 Dec 2017 10:00:20 GMT

It would be useful topProvide a summary if it works out. People would normally follow the conventional route of trying to 'fix' the dem by filtering ad nauseam to 'smooth' data. In the process the nature of the shapes in the 'larger' areas get altered as well

Re: Isohypses - filtering out very small areas of elevation?

Michèlevon_Kocemba — Mon, 11 Dec 2017 10:18:12 GMT

Solution as suggested by
Dan_Patterson‌
I took a look at the attribute table of the contour layer, inspected the length of the contours and - by comparing with my dem that I have already classified and altered according to my needs - which values could be deleted. Using the selection tool I selected those below a specific value and created an extra layer with only those contours remaining that I wanted to keep.

Needed to do that quite carefully since there were some peaks that I wanted to keep, did the selection tool with the additional condition of regarding the contour value so that mountains would not vanish, but besides that this worked fine!