topic Re: Reduce points in Point feature from a 30 meter DEM raster in Python Questions

Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Mon, 09 Nov 2015 23:48:18 GMT

I have created a point feature from a 30 meter DEM raster and I would like to reduce the number of points (decimate) - the number of points are too large to manage. Is there a toolbox for this or can this be done with the select attribute tool. Thanks.

Re: Reduce points in Point feature from a 30 meter DEM raster

DanPatterson_Retired — Tue, 10 Nov 2015 01:05:12 GMT

probably many options, but you could add a field to the table and calculate a random number for the field , then select the number of features you want. If you want to do a rondom-y stratified-ish sample, you could calculate add a field and use the field calculation on either the X and/or Y

!shape!.centroid.X % 2 ( I think) which will calculate a binary value indicating whether the coordinate is even or odd...then query out your choise and save them. So decide whether you need random, random with a minimum number of samples, points with a minimum spacing etc etc.

Field calculations are here if you need examples. Calculate Field examples—Help | ArcGIS for Desktop

Re: Reduce points in Point feature from a 30 meter DEM raster

DuncanHornby — Tue, 10 Nov 2015 15:12:51 GMT

I don't think this is really a Python question, more of a geo-processing problem (python being one solution).

Assuming you have a Shapefile then your row id is "FID". The following Where clause can be used in the Select by Attribute and it will select the first and every tenth row:

"FID" / 10 = round( "FID"/10,0) or "FID" = 1

If your dataset is a geodatabase feature class then substitute FID with ObjectID.

You can then either delete these or invert the selection and delete out the batches of 9 rows. If the second option is what you want it will be quicker to simply export the selected 10th rows.

Re: Reduce points in Point feature from a 30 meter DEM raster

DanPatterson_Retired — Tue, 10 Nov 2015 15:16:28 GMT

Duncan, I didn't mean to suggest it was a python problem, however, python in the field calculator etc allows for builtin random module access making random number generation easier etc.

Re: Reduce points in Point feature from a 30 meter DEM raster

DuncanHornby — Tue, 10 Nov 2015 15:28:30 GMT

Dan, I was not thinking your answer was unsuitable more that the original question is located under the python discussion board and that it is seems more suited to the general geo-processing page. If I had the power I would be moving it into that section.

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Tue, 10 Nov 2015 15:36:57 GMT

Duly noted but I was originally considering applying a Python script to execute this. I couldn't figure out the code to use in 'select by attribute' to delete rows in the table.

Thanks for your help.

Re: Reduce points in Point feature from a 30 meter DEM raster

DuncanHornby — Tue, 10 Nov 2015 15:48:45 GMT

Chris, If Dan's or my suggestion solves your problem you should tick it as such, shows to others a solution was found and we get lots of lovely points!

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Tue, 10 Nov 2015 16:30:55 GMT

Duncan,

I'm going to try this. I assume I need to start an edit session in order to delete the records in the attribute table?

Re: Reduce points in Point feature from a 30 meter DEM raster

DuncanHornby — Tue, 10 Nov 2015 17:21:05 GMT

You can or you can simply use the Delete Features geoprocessing tool! A warning about this tool, if no selection exists then it Deletes EVERYTHING and there is no undo for this tool! So if you are building a model to run this many times it would be sensible to put in a GetCount and use that as a precondition to the Delete Features tool, otherwise you may end up shooting yourself in the foot.

Re: Reduce points in Point feature from a 30 meter DEM raster

IanMurray — Tue, 10 Nov 2015 17:49:20 GMT

Another option is to simply resample the raster down to an even lower resolution before converting to points.

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Tue, 10 Nov 2015 18:16:04 GMT

That's perfect. How do I resample the raster?

Sent from my iPhone

Re: Reduce points in Point feature from a 30 meter DEM raster

IanMurray — Tue, 10 Nov 2015 19:20:18 GMT

See this tool help.

ArcGIS Help 10.1

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Tue, 10 Nov 2015 23:20:12 GMT

Ian,

It looks like this will work!

Thanks.

Re: Reduce points in Point feature from a 30 meter DEM raster

XanderBakker — Wed, 11 Nov 2015 18:06:59 GMT

What interests me most, is why you converted the raster to points? What is your goal with that? Do you want to mimic a point cloud or is there a specific analysis you plan to do on the points?

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Thu, 12 Nov 2015 00:22:26 GMT

Xander,

I have to create a simple xyz flat file that I can grid in another software program and by converting the raster DEM into a point file I already have the z portion - I just needed to add the xy coords and export out as a csv which I can bring into the program. The problem was the software program was choking on the amount of points generated from a 30m DEM.

Re: Reduce points in Point feature from a 30 meter DEM raster

DanPatterson_Retired — Thu, 12 Nov 2015 00:56:48 GMT

I am sure you are aware of the potential effects of resampling to a lower resolution and these should be addressed or considered before using the results as inputs to other processes. For the sake of completeness, the discussions given in the help are useful....especially for those that just accept the defaults as a matter of course.

Resample—Help | ArcGIS for Desktop

Aggregate—Help | ArcGIS for Desktop

Re: Reduce points in Point feature from a 30 meter DEM raster

XanderBakker — Thu, 12 Nov 2015 01:25:28 GMT

Hi Chris, thanks for the additional information. It makes sense that a software package chokes on a CSV file if it contains a large amount of records (which it will probably have). Resample is a simple way of reducing the amount of data but will result in loss of quality as Dan mentions. If there is no other exchange format that ArcGIS can write to and the other software can read from, and the extent has been reduced to a maximum, here is not much left to do.

Re: Reduce points in Point feature from a 30 meter DEM raster

curtvprice — Thu, 12 Nov 2015 04:16:04 GMT

If you have 3D analyst you can convert to a TIN with a tolerance, the TIN can then be exported to points, which then can be re-gridded by your external program. This approach would allow you to preserve more detail than simply using Resample (or, better, Aggregate) to create a coarser regularly-spaced raster.

Re: Reduce points in Point feature from a 30 meter DEM raster

ChrisMartin9 — Thu, 12 Nov 2015 14:37:13 GMT

Xander,

I do have 3D analyst but I have never done this. Do you have more information on how to convert the raster to a TIN and then export to points?

Thanks.

Re: Reduce points in Point feature from a 30 meter DEM raster

GerryGabrisch — Thu, 12 Nov 2015 17:24:20 GMT

Or you could create a new attribute (call it modulo) and populate it with a modulo value using the Object ID and a value of say 10, then select all the records with a modulo value of 0 and and export only those records ( 1/10th of those originall records) to a CSV. That way you could preserve the original point values of the 30 m DEM and not end up with the point values of the resampled surface.