Problems measuring distance, calculating acreage, and incorrect scale

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08-12-2017 12:18 PM
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New Contributor II

Inserting the scale bar with USGS topographic basemap clearly shows the map scale is more than twice what is expected. Measure a section (square mile) with the measure tool yields nearly 3000 acres when it should only by 640. Similarly measuring the square miles of a single section should be only 1 square mile but measure tool gives 4. An inserted scale bar is also off by a similar factor. Any suggestions on why ArcMap 10.2.2 is miscalculating? 

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MVP Esteemed Contributor

Is the coordinate system on your data frame consistent with the the datasets your layers point to?  Are you displaying data, for example, in an inappropriate projection (ie trying to use UTM Zone 18 in Massachusetts).

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MVP Esteemed Contributor

check units... don't assume they are in feet, especially if you didn't download directly from the site and check metadata to confirm.  Also confirm that the units are indeed planar and not in decimal degrees, regardless of what your dataframe is set to.

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MVP Esteemed Contributor

What's the spatial reference of your map? If you want to measure meters it should be a projected coordinate system (like UTM). Also check the data frame properties / General tab to see how the distance units are set.

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New Contributor II

Units under Data Frame Properties are:

Map: Meters (seemed to be locked in, can't change it)

Display: Decimal Degrees

Reference Scale: Use Current

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New Contributor II

WGS_1984_Web_Mercator_Auxiliary_Sphere
Projection: Mercator_Auxiliary_Sphere
Linear Unit: Meter (1.0)

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MVP Esteemed Contributor

What part of the world are you displaying? This projection is awful in polar regions.

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New Contributor II

Just north of Anchorage, Alaska.

Switched Map Project of Data Frame to following and this has resolved the issue:

NAD_1983_NSRS2007_UTM_Zone_6N
Projection: Transverse_Mercator
False_Easting: 500000.0
False_Northing: 0.0
Central_Meridian: -147.0
Scale_Factor: 0.9996
Latitude_Of_Origin: 0.0
Linear Unit: Meter (1.0)

Many thanks guys!

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MVP Esteemed Contributor

FYI - more about why Web Mercator is handy and convenient for maps on your phone and map services, but not so much for analytical use!

Measuring distances and areas when your map uses the Mercator projection | ArcGIS Blog  

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New Contributor II

Trying to measure acreage, but I check different units using both line and polygon switching from meters to miles, etc. all appear to be consistently off. All other layers, including waypoints, tracks, and trails overlay correctly on Basemap. Grid coordinates appear accurate as well. Only scale and measure tools appear off. 

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MVP Esteemed Contributor

Is the coordinate system on your data frame consistent with the the datasets your layers point to?  Are you displaying data, for example, in an inappropriate projection (ie trying to use UTM Zone 18 in Massachusetts).

View solution in original post