topic [Cross-post Python forum] Finding saddle points in a grid? in ArcGIS Spatial Analyst Questions

[Cross-post Python forum] Finding saddle points in a grid?

StefanHaglund1 — Wed, 19 Dec 2012 10:22:47 GMT

Sorry for the cross-post, not sure where this belongs.

See http://forums.arcgis.com/threads/73854-Finding-saddle-points-in-a-grid

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Hello all!

I would like to be able to identify saddle points in my grid.

Does anyone know of a method to do this? Search has led me in the way of Numpy. Would that be the right direction to go or can this be achieved with Spatial Analyst?

Thanks!

Re: [Cross-post Python forum] Finding saddle points in a grid?

SebastianSantibanez — Thu, 20 Dec 2012 16:40:41 GMT

Numpy seems right. take a look a this
http://homepages.math.uic.edu/~jan/mcs275/arrays.pdf
it has the right algorithm ready for you

Re: [Cross-post Python forum] Finding saddle points in a grid?

curtvprice — Thu, 20 Dec 2012 20:16:24 GMT

I would like to be able to identify saddle points in my grid.

Have you tried the Curvature tool? The hydrology tools (Flow Accumulation) may also be get you where you need to go.

Re: [Cross-post Python forum] Finding saddle points in a grid?

StefanHaglund1 — Fri, 21 Dec 2012 07:22:48 GMT

Thanks Sebastian, I'll take a look at that.

Curt, I see you want a better definition of a saddle point. Please have a look at this link http://en.wikipedia.org/wiki/Saddle_point.

I geographical terms, I think of the saddle point as a mountain pass like in the second picture in the link. From that point, the surface curves up in one direction and down in another.

In more mathematical terms, this means that the point will be the minimum value in its row and maximal in its column. This, I believe, is only valid for a smaller neighborhood. If you consider a larger area like most DEM's, you could potentially have a min value in a row in a sink, i.e. the surface curves upwards in all directions around it. At the same time you can have a "true" saddlepoint in the same row somewhere else in the DEM.

This means I am not sure how I can adapt the solution from Sebastians suggestion to take a larger area into account. Or I am thinking in the wrong direction here?

Thansk!

Re: [Cross-post Python forum] Finding saddle points in a grid?

SebastianSantibanez — Fri, 21 Dec 2012 16:34:25 GMT

I guess that you could fit some helmets to your geographical features and obtain your saddle point by analyzing the deterministic surface you would obtain from that fitting.
Another suggestion I would explore is passing a low pass filter through your raster so your geography gets smoother.

Re: [Cross-post Python forum] Finding saddle points in a grid?

curtvprice — Fri, 21 Dec 2012 22:10:14 GMT

I geographical terms, I think of the saddle point as a mountain pass like in the second picture in the link. From that point, the surface curves up in one direction and down in another.

You may want to look at the Curvature tool. Comparing that value with some other measures (slope for example) may help you isolate the area you're interested in.

Re: [Cross-post Python forum] Finding saddle points in a grid?

MapMan1 — Tue, 01 Mar 2022 18:39:25 GMT

Stefan, I am trying to find/locate saddle points in a DEM as well. Did you ever find a solution? If so, can you point me to it or describe it to me? You can email me at jed.flint -at- yahoo.com