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Thread: Extract by mask changes the values

  1. #1
    Dean Mansfield
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    Question Extract by mask changes the values

    Hello

    I have a large number of large ascii files and I need the centre values keep the same and the outer areas made NoData (outer values are water, inner values are land). I run the tool 'ASCII to Raster' then 'Extract by Mask' then convert back to ascii with Raster to ASCII.
    When I use the Extract by Mask tool it works for the NoData task but the values in some inner cells change, similar yes, but have changed. Why does this tool change the values and how do you stop this?
    I have tried many other ways to do the task, but can't even get close, it really doesn't seem like a complex task, it just won't work. Please help

  2. #2
    Steve Lynch

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    Default Re: Extract by mask changes the values

    Dean

    What do you mean by changed?

    Can you give a before and after example?

    Also remember that the raster will be 32 bit floating point precision if your as using the FLOAT option. This means that not all values can be represented exactly.

    1.3 for example is represented as 1.2999999523162841796875E0

    Hope this helps
    Steve

  3. #3
    Dean Mansfield
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    Default Re: Extract by mask changes the values

    Hi Steve

    I can send you an actual file if you want? Some values are out by up to 2-3% of the original value (ie 142 becomes 169)

    This tool/process I'm using seems to sample it and comes back with different values.

    I want the values to be exactly the same, just with an area around the outside as NoData.

    Cheers

    Dean

  4. #4
    Steve Lynch

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    Default Re: Extract by mask changes the values

    Dean

    Please zip it and send it to slynch@esri.com

    Thanks
    Steve

  5. #5
    Steve Lynch

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    Default Re: Extract by mask changes the values

    The extents of the 2 raster were different. In many cases the neighboring cell was being queried.

    Steve

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