Sink results in very different demarkations depending on the resampling of the raster data. Could someone explain why? I don't understand this?

In the attached images you could see the differences. I use elevation data with a resolution of 5 meters. Sink06m ... is in my opinion better because nearly all lakes are demarked as sinks. Sink06m is resampled to a cellsize of 6m biliniear. Sink10m is resampled to a cellsize 10 meters, but the results are totally different?

Am I doing something wrong?

This is how I calculated the areas:

`import arcpy, os`

from arcpy.sa import *

arcpy.env.overwriteOutput = True

arcpy.env.workspace = r"MyPath\DGM.gdb"

# Local variables:

DGM005_tif = r"MyPath\DGM005_3.tif"

Fill_tif1 = "Fill_1"

resample = "resample"

FlowDir_Fill2 = "FlowDir_1"

Sink_FlowDir1 = "Sink_FlowDir1"

absenkung = "Absenkung"

arcpy.CheckOutExtension("Spatial")

outCon2 = Con(Raster(DGM005_tif) > 0, DGM005_tif)

# Process: Füllung

arcpy.gp.Fill_sa(outCon2, Fill_tif1, "")

print("fill")

arcpy.Resample_management(Fill_tif1, resample, "8", "BILINEAR")

print("resample")

# Process: Fließrichtung

arcpy.gp.FlowDirection_sa(resample, FlowDir_Fill2, "NORMAL", absenkung)

print("flowdir")

# Process: Senke

arcpy.gp.Sink_sa(FlowDir_Fill2, Sink_FlowDir1)

print("sink")

arcpy.RasterToPolygon_conversion(Sink_FlowDir1, "{}{}".format("Senke08mfill", os.path.basename(os.path.splitext(DGM005_tif)[0])), "SIMPLIFY",

"VALUE")

print("Done")

arcpy.CheckInExtension("Spatial")

Hi Johannes,

Thanks for clarifying your analysis objectives. I will try to outline a few steps, where some are required and some other might be optional. Since there are many different ways to approach each step, I will just pick one, however, you can decide to do run some other tool to find the same final answer.

I am still not clear why you want to change the resolution of your DEM, so for the moment I will put it aside and approach this analysis with the original data. And then we will come back to changing the resolution if needed.

1. Smoothing out the DEM to remove small errorsIf you are worried about the accuracy of the data and want to smooth it out, you can use the Filter tool with the 'Low pass' option. If you want more control over the neighborhood, you can use the Focal Statistics too.

2. Removing small sinksSince you are interested in larger sinks, surface area more that 5000 sq.m, you may decide to remove some sinks at this early stage of analysis. You can do this by running the Fill tool with a z-limit.

The z-limit specifies the maximum difference allowed between the depth of a sink and the pour point and determines which sinks will be filled and which will remain untouched. The z-limit is not the maximum depth to which a sink will be filled.Choosing an appropriate z-limit could be tricky when your criteria is the surface area of a sink. However, since you are looking at rewetting and dam, I am sure you have a depth of the sink in consideration below which the sink is not worth considering. So, you can easily use that depth to weed out some sinks.

Let's call this filled (with z-limit) DEM as

FilledDEMwithZLimit. We will use this DEM for further analysis.3. Identify remaining sinksNow it's time to identify the remaining sinks in

FilledDEMwithZLimit.Run the Fill tool using the D8 option and then run the Sink tool.

The output from the Sink tool will contain all remaining sinks. Let's call this output

AllSinks.Please make sure the output

AllSinkshas the raster attribute table. If not, please use the Build Raster Attribute Table tool to create the attribute table.4. Identify sinks larger than 5000 sq.m.There are many ways to approach this step. I will start with a very simple approach. We will just look at the COUNT field in the raster attribute table of

AllSinksto look for sinks that meets your criteria.You said your DEM cell size is 30cm. So, any sink with a cell count of (5000 / (0.3 * 0.3)) = 55,556 will meet your criteria.

You can look at the raster attribute table of

AllSinks,select rows where COUNT is equal to or greater than 55,556. These are the sinks meet your criteria.An alternative approach is to run Zonal Geometry As Table to find out which sinks meet your criteria. The good part is, from the output of Zonal Geometry As Table you will not only get the area, you will have some additional information about the shape and location of the sinks that meet your criteria.

5. Finding the total sink area for the sinks meet your criteriaYou can use the Summary Statistics tool to do this task.

You can use the

AllSinks,select the rows that meet the criteria, run the Summary Statistics tool and find Sum on the COUNT field. You can multiply the sum of COUNT by (cellsize * cellsize) to find the total area.Alternatively you can use the output from Zonal Geometry As Table , select the rows that meet the criteria, run the Summary Statistics tool and find Sum on the AREA field.

An alternate approachPlease note, there are many ways we can come to the same result. One of the interesting approach could be to to run Fill without the z-limit using

FilledDEMwithZLimitas input. Let's call this output asFilledDEM. Then use the Cut Fill tool to find the area and volume of each sink. You can then select your sinks by applying the criteria of the Cut Fill output. And then of course run the Summary Statistics tool to find the total area.Changing the resolution of the DEMNow that we have discussed the steps for fining your sinks and calculating the total area, let's go back to the DEM resolution question. I am still not sure why you need to lower the resolution of the DEM if your computer can process all the steps with the original high resolution DEM within reasonable time. If you still think it is necessary to lower the resolution of your DEM, you can run the Resample tool.

In my opinion, you can do it either before step 1 or after. But before you run any fill operation in step 2.

Hope this help!

Please let me know if you have any additional questions.

Thanks!

Noman

Esri, Redlands, CA