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Thread: Digitizing errors

  1. #1
    Andy New
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    Default Digitizing errors

    Hi there,

    I am relatively new to arcmap. At the moment I am trying to perform what seems like a simple task of digitizing lakes from aerial imagery to calculate their surface area. I have been using the arcgis online MDA NaturalVue Satellite Imagery as it has extremely good resolution for many of the sites I want to use. I have to digitize just under 600 lakes from across the entire northern hemisphere so it seems a good base map to use. However, every time I attempt to do this by using polygons the results that come out for the area are nowhere near correct. I have tried adjusting the coordinate systems but I don't seem to be able to get what seem like reasonable results? I must be missing something simple?

    Could anybody offer me any advice for calculating lake surface area from one of the arcgis online maps (perhaps the most appropriate basemap to use), and the relative dataframe/coordinate systems that I need to set my polygon shapefiles to etc?

    I know this probably sounds simple but for some reason I just cannot get it to work...

    Many thanks
    Andy

  2. #2
    Hardolph Wasteneys
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    Default Re: Digitizing errors

    Andy,

    Use North_America_Albers_Equal_Area_Conic projection for your digitizing file if the lakes are all over the continent.
    WGS_1984_Web_Mercator_Auxiliary_Sphere for the file will give you very high areas increasing as you go north because of the cylindrical stretching distortion, but should be OK for the data frame although you can change that to Albers as well if you like. It should not change the areas.

    If you use this in a geodatabase feature class the area is automatically calculated in a field.

    Hardolph
    Hardolph Wasteneys Ph.D., P.Geo.
    Geologist
    Vancouver Island

    hardolph@bravewolf.com

  3. #3
    Andy New
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    Default Re: Digitizing errors

    Hi Hardolph,

    Thank you very much for getting back to me with your advice, it has worked perfectly from what I can see so thanks for your input.

    All the best
    Andy

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