DEM Comparison in ArcMAP

[GlenKilpatrick1]

Hi,

I am doing an undergraduate project on the use of LIDAR data for DEM generation and flood modelling.  I currently have 2 LiDAR datasets for a broad area and a survey data set for a much narrower area within that.  These data sets were captured at times before and after a major flood event.  I need to carry out a series of comparisons of heights between the DEMs (average height difference between DTMs, a map showing the scale of elevation differences across the subject area).  What would be the best geoprocessing functions to use for this purpose (I have created terrain datasets but ideally I would be working in raster format)?  Also is there any way of checking up on the progress of a geoprocessing function prior to its completion? Just sometimes I execute a function but there is no evidence on the screen as to what is happening to a while later when it is completed.

Regards,

Glen Kilpatrick

[JayantaPoddar]

Hi Glen,

Try converting Terrain to Raster (3D Analyst)​. Also make sure the tool parameters are same for conversion of both the terrain datasets.

Then, Using Raster Calculator,

Float ("Post_Flood" - "Pre_Flood")

For keeping a check on the Geoprocessing status, TURN OFF background processing.

In ArcMap, Geoprocessing > Geoprocessing options. Uncheck "Enable" for background processing.

[ReneeBrandt · Accepted solution]

Hi Glen.

I checked with the team here on your question, and received the following response:

1. Agreed, don't use a terrain – that duplicates data and (as noted) is still not a DEM.   Instead, use a LAS Dataset to reference the LAS Files, then if you want a DEM on disk, you can run “LAS Dataset to Raster”
   1. There’s a better version that creates TILES but you have to download from http://links.esri.com/3dSamples --> inside see “LAS Dataset to Tiled Rasters”
2. For MUCH more information re: recommendations and best practices (but warning, it’s long) see http://esriurl.com/LidarGuidebook

As for checking on status of GP, they also mentioned that you may be running in the background (default and they prefer to run GP tools in the foreground. To do that, go to:  GeoProcessing, then GeoProcessing Options, then Background Processing: DESELECT the “enable” box.

Also, some advice you may want to take:  Depending on the tool (every one is different), it may not tell him very much during execution but at least he’ll see the verbose output.  If he’s crunching a bunch of lidar and waiting a long time, most tools will support the environment setting “processing extent” and I’d set it to “Same as display”, then zoom in to a small section of a dataset and run a test first to be sure the tool is giving him what he wants before running a large dataset.

Back to 1)a., if you're creating a DEM and the extent is really large, you may have problems creating a single raster.  In that case, use the “LAS Dataset to Tiled Rasters” tool.  It scales better for large datasets

Hope that helps, and good luck with your studies!  Let us know how it goes!

[MikeCusi]

use Geoprocessing | Results to show the results window. This will give you some idea what is happening while a geoprocessing tool is running. All the messages will be shown. This is useful particularly if you have background processing ON.

If you turn background processing OFF the dialog box that show what is happening will stay open while it runs.

Also when comparing rasters, make sure one of them (usually the one with a larger extent) is used as your SNAP raster when you create the rasters.

[GlenKilpatrick1]

Thanks for the assistance Jayanta, Renee and Mike.  I was able to successfully create a DEM of difference.  Will utilise the advise and ask questions if I have further queries.

Regards,

Glen.

[GlenKilpatrick1]

Hi,

I have another question about raster data.  I have created a difference raster dataset by using the raster calculator as above.  This is to calculate LiDAR accuracy by comparing a LiDAR survey raster DEM to a field survey raster DEM.

However this analysis only applies outside of the water.  I have polygons delineating the water and I need to find a way of clipping out the water sections of the raster or removing the values of these cells.  The purpose for this is to be able to generate raster summary statistics for the area excluding water.  Is there a function that can remove or null a small portion out of a raster data set?

Regards,

Glen

[DanPatterson_Retired]

Check out 'setnull' in the spatial analyst extension.  It is one approach to masking out data prior to - or after a process.