topic Re: Measurement Tool Error? in ArcGIS Online Questions

Measurement Tool Error?

StephenKnowles — Wed, 12 Dec 2012 14:26:13 GMT

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.

Re: Measurement Tool Error?

StevenMoore1 — Wed, 12 Dec 2012 14:50:17 GMT

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

http://support.esri.com/en/knowledgebase/techarticles/detail/39404

The Bing and Esri basemaps all use WGS84 Web Mercator.

Re: Measurement Tool Error?

StephenKnowles — Wed, 12 Dec 2012 18:00:53 GMT

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.

Re: Measurement Tool Error?

KenBuja — Wed, 12 Dec 2012 18:10:43 GMT

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.

Re: Measurement Tool Error?

MelitaKennedy — Wed, 12 Dec 2012 22:26:18 GMT

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