Areas in GM

geomanniegeomannie Global Mapper UserPosts: 161Trusted User
edited September 2014 in Technical Support
I am using GM to calculate areas for some large tracts of land. The polygons I am using are over 200km long and over 11,000km2 in area.

I am keen to know the exact areas of these polygons as the area reported by GM is different to that specified in the official goverment document (which I know to contain some errors). I have checked the area of the polygons (defined by a series of points) in ArcGIS and get another value, not the same as the GM value or the official document. Further confusion is generated when I flip in GM from UTM projection to geographic lat-long. For example, the area of one polygon goes from 11,231km2 in UTM to 11,303km2 in geographic, but only under the geometry feature information panel. The enclosed area attribute remains at 11,231km2.

I am now thoroughly confused as to the correct areas of these polygons. Could it be that earth curvature is coming into play here? Is GM assuming that we are dealing with a simple flat plain? Can GM report area incorporating curvature? Any pointers as to how GM deals with area would be much appreciated.

Many thanks

geomannie

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2011
    geomannie,

    In order to get results consistent with other applications, like ArcGIS, if your current projection is set to a projected system, like UTM, the area of the feature is calculated using the projected system coordinates, even through they will have a bit of distortion. Typically the better a projection that you choose for your area of interest the more accurate your area calculation will be. If your projection is set to Geographic (lat/lon) or a severely distorted projection like Mercator, then a lat/lon-based calculation will be used that should be pretty accurate, although for very large areas where the earth's curvature within the area is significant the area may be slightly under-reported. This is a common problem in most applications as area calculations involved the ellipsoidal curvature can be quite complex.

    The ENCLOSED_AREA attribute is not updated when you change projection, only when you add/update feature measure attributes. The Geometry field in the Feature Info dialog on the other hand always updates.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • geomanniegeomannie Global Mapper User Posts: 161Trusted User
    edited February 2011
    Hi Mike

    If I understand you, depending on the size of the area, there may be uncertainty in the precision of calculated areas due to earth curvature. As you say the maths to do area calculations involving ellipsoidal curvature can be quite complex.

    In my particular circumstance I have a polygon given as ~0.05degree points located within 10degrees of the equator. Distortion will be minimal. What would give me the most accurate area? Creating an uprojected lat-long polygon on which to measure area, or flipping the polygon into UTM or other projection and then measuring the area?

    Many thanks

    geomannie
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2011
    You should get the best results by setting the current projection to a custom one with a center inside your polygon, like perhaps a Transverse Mercator or Orthogonal projection with the Central Meridian and Origin Latitude around the center of the polygon, although UTM should be pretty good too unless the area is far from the center of a UTM zone.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Kuldip UpasaniKuldip Upasani Global Mapper User Posts: 6
    edited August 2011
    Dear Sir,
    I have inserted two overlapped polygons in image to locate some area, is polygon 'A', polygon 'B'.
    Now I can able to get the information regarding to the area by using the feature info tool. but If I want to know the perimeter & area of overlapped area???
    Kindly refer the attached image...
    Thanking you...
    Kuldip U.
    Maharashtra, India
    Global_Mapper_Query_screenshot.JPG
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2011
    Kuldip,

    What you need to do is select both areas with the Digitizer Tool, then right-click and select the option to find the intersection of the areas. You will then have a new area that is the intersection of those two that you can select with the Feature Info tool to see the perimeter and enclosed area.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • QiaoQiao Global Mapper User Posts: 73Trusted User
    edited September 2014
    Hi Mike,

    I am wondering whether you could give us some descriptions of the "lat/lon-based calculation" you mentioned in your first reply when the data is with a geographic system. We are looking for a way of area calculation which is suitable for state-wide data with NAD83 Geo system and currently trying to understand the differences among different approaches.

    Thanks in advance,

    Qiao
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