Hydrology Tools - Flow Accumulation Raster Attribute Table

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05-04-2018 06:29 AM
MichaelCullen
New Contributor II

I have 2 DEMs of a portion of Eastern PA in ArcMap:  one is a large area in Pennsylvania another DEM is a smaller clipped polygon within the larger one.  When I try to conduct Hydrology Tool analysis (Fill, Flow, and accumulation) the raster I generate in the TOC for the larger polygon area does not allow me to access an attribute table.  For the flow accumulation raster I selected INTEGER not FLOAT per my lab instructions.  My project requires me to access the table to calculate the number of cells accumulating to a surface area.  I read on another thread that ArcGIS will grey out the attribute table if the raster accumulation tool has too many classes to calculate.  I am attempting to try the process on the smaller polygon to access the attribute table.  What is the class number cutoff that ArcMAP has for prohibiting access to the attribute table?  Is there a threshold area for a polygon after which it becomes disabled?  

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5 Replies
ChrisDonohue__GISP
MVP Alum

Are you using ArcGIS Pro or ArcGIS Desktop?  I see that the post is tagged with ArcGIS Pro, but just wanted to be sure as there are some differences in how ArcGIS Pro does the processing compared to ArcGIS Desktop.

My first thought on what is going awry (regardless of whether using Pro or Desktop) is that something went amiss in the processing of the data leading up to the Flow Accumulation tool.  Review each of the process steps you did and check the outputs of each process to see if the results are expected.  Also, be sure that all the needed steps/processes are being followed.

ArcGIS Help (10.2, 10.2.1, and 10.2.2) 

Deriving runoff characteristics—Help | ArcGIS Desktop 

Chris Donohue, GISP

MichaelCullen
New Contributor II

Thanks Chris for the process diagram!   I am using ArcMAP (desktop).  I started a depressionless DEM, and then proceeded to executethe Hydro Arctoolbox algorithms to generate FILL.img (input raster set to original depressionless DEM.img, FLOWDIRECTION.IMG (input raster set to FILL.img), and then finally FLOWACC.IMG (input raster set to FLOWDIRECTION.img).  In other threads I heard that there is a maximum size for a DEM after which the classes become too many and count be counted, thus the greying out of the attribute table of this raster layer.  I am trying to calculate the total accumulation of water (in acres or sq miles) overlayed on top of 5 separate Landsat scenes I have loaded in the .mxd.

Thanks!

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DanPatterson_Retired
MVP Emeritus

You mention float and integer.  Floating point grids/rasters have no table, only integer grids/rasters do.

MichaelCullen
New Contributor II

Yes, I've set the option to INTEGER in the lower portion of the tool's dialog box.

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DanPatterson_Retired
MVP Emeritus

go to C:\Program Files (x86)\ArcGIS\Desktop10.6\Utilities\AdvancedArcMapSettings.exe crank it up and you can view/change the value there for the max number of records, I can't remember off hand, but it is large