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Thread: Measurement Tool Error?

  1. #1
    Stephen Knowles
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    Default Measurement Tool Error?

    When I load a basemap (e.g. BingMapsAerial), and perform distance measurements, I find that the returned values are almost 50% too high. (to determine this, I zoomed in on an American football field and measured the distance between goal posts (120 yards, 360 feet), and get ~476 feet as the return value. I found this same error on other basemaps that appear when clicking "add basemap", and using different reference distance techniques. Does anyone know what the source of this error is? Since it appears to be a constant error, I just multiply any measurement I make by 0.755, but that's a little disconcerting.

  2. #2
    Steve Moore
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    Default Re: Measurement Tool Error?

    Take a look at this Knowledge Base article, I think it should help answer your questions:

    http://support.esri.com/en/knowledge...s/detail/39404

    The Bing and Esri basemaps all use WGS84 Web Mercator.

  3. #3
    Stephen Knowles
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    Default Re: Measurement Tool Error?

    When dealing with several miles of distance, I could believe a projection could cause a significant measurement error, but this error is manifested as a tremendous error (~50%) over a very short distance (120 yds). Even if dealing with english vs. metric, a 50% error wouldn't result (1m vs 1yd), so I am still baffled, but thanks for the input.

  4. #4
    Ken Buja

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    Default Re: Measurement Tool Error?

    Web maps usually use Web Mercator projection, which is generally bad for doing measurements. Take a look at this blog posting, which goes into much greater detail about this. The post has a link to an application that shows the difference when using a Web Mercator projection and ones more suitable for length and area.
    Last edited by kenbuja; 12-12-2012 at 11:13 AM.
    Ken Buja
    Silver Spring, MD

  5. #5
    Melita Kennedy

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    Default Re: Measurement Tool Error?

    The scale distortion for Mercator based on a sphere (which web Mercator is) varies as 1/cos(latitude). The equation when based on an ellipsoid is a little more complicated. So if at 40N, 1.3054.

    Melita
    Melita Kennedy
    Sr. Esri Product Engineer

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