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How To Calculate Average or Median Elevation Values For A Grid

Question asked by HMArnold on Oct 13, 2019
Latest reply on Oct 15, 2019 by HMArnold

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I'm using ArcMap Desktop 10.7 for flood analysis using GEOTiff ground elevation rasters.

My hope is to be able to define a grid, for example 10 meters by 10 meters, then recover an average or median elevation value for all raster values in that fall within that cell.

Essentially dividing the resolution of the original resolution into a number of cells I can work with individually.

Ending up with a table of values would be ideal.

I'm expecting it to be a two step process, the first being the definition of the 10 by 10 grid, the second being the calculation of the average or mean elevations for each cell.

Any suggestions appreciated.

MervynLotter Oct 14, 2019 6:14 AM

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Hi Hank

I think I understand. You should then try the Block Statistics tool that is available in the same toolbox. See How Block Statistics works—Help | ArcGIS Desktop and Block Statistics—Help | ArcGIS Desktop Or you

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MervynLotter Oct 13, 2019 9:27 AM

If you want to calculate the average or median value of an elevation raster, then look at Focal Statistics. Focal statistics can provide you with a multitude of various mathematical operations for, example, a 9 x 9 pixel area. See How Focal Statistics works—Help | ArcGIS Desktop and Focal Statistics—Help | ArcGIS Desktop

Or if your zones are in a different raster, then look at using Zonal Statistics as Table How Zonal Statistics works—Help | ArcGIS Desktop and Zonal Statistics as Table—Help | ArcGIS Desktop

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HMArnold @ MervynLotter on Oct 14, 2019 6:16 AM

Mervyn,

This looks like an interesting tool, and can be used to even out inconsistencies in an GEOTiff, but what I'm looking for is a way to reduce the resolution of an elevation dataset, or in essence pixelate it.

In other words, if my elevation TIFF is 1000 pixels by 1000 pixels, and I calculate means of each 20 by 20 cell, the resulting cells would have dimensions of 50 by 50.

Each of those cells would be a single mean value of what was a 20 by 20 area of the source.

For flow calculations, 50 by 50 grid would take significantly less calculation power than the original 1000 by 1000 source. Since the resolution would also be lower, the calculations would only be an approximation - but with a rational calculation requirement.

I hope this makes sense

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MervynLotter @ HMArnold on Oct 14, 2019 6:14 AM

Hi Hank

1 person found this helpful

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HMArnold @ MervynLotter on Oct 15, 2019 7:15 AM

Block Statistics was exactly what I was looking for.

It will take some time to come up with the right grid size, but when you add X,Y coordinates and then export, you get a reduced resolution grid of the elevation values

Thanks very much

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MervynLotter Oct 13, 2019 9:27 AM
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If you want to calculate the average or median value of an elevation raster, then look at Focal Statistics. Focal statistics can provide you with a multitude of various mathematical operations for, example, a 9 x 9 pixel area. See How Focal Statistics works—Help | ArcGIS Desktop and Focal Statistics—Help | ArcGIS Desktop

Or if your zones are in a different raster, then look at using Zonal Statistics as Table How Zonal Statistics works—Help | ArcGIS Desktop and Zonal Statistics as Table—Help | ArcGIS Desktop
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- HMArnold @ MervynLotter on Oct 14, 2019 6:16 AM
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  Mervyn,
  
  This looks like an interesting tool, and can be used to even out inconsistencies in an GEOTiff, but what I'm looking for is a way to reduce the resolution of an elevation dataset, or in essence pixelate it.
  
  In other words, if my elevation TIFF is 1000 pixels by 1000 pixels, and I calculate means of each 20 by 20 cell, the resulting cells would have dimensions of 50 by 50.
  
  Each of those cells would be a single mean value of what was a 20 by 20 area of the source.
  
  For flow calculations, 50 by 50 grid would take significantly less calculation power than the original 1000 by 1000 source. Since the resolution would also be lower, the calculations would only be an approximation - but with a rational calculation requirement.
  
  I hope this makes sense
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  - MervynLotter @ HMArnold on Oct 14, 2019 6:14 AM
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    Correct Answer
    Hi Hank
    
    I think I understand. You should then try the Block Statistics tool that is available in the same toolbox. See How Block Statistics works—Help | ArcGIS Desktop and Block Statistics—Help | ArcGIS Desktop Or you
    1 person found this helpful
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    - HMArnold @ MervynLotter on Oct 15, 2019 7:15 AM
      Mark Correct
      Correct Answer
      Block Statistics was exactly what I was looking for.
      
      It will take some time to come up with the right grid size, but when you add X,Y coordinates and then export, you get a reduced resolution grid of the elevation values
      
      Thanks very much
      Like • Show 0 Likes0
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