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Santa causing meridian over the North Pole to distort

JakeJake Global Mapper UserPosts: 276Trusted User
edited December 2012 in Bug Report
Windows 7 64 bit GM 13.2.2 Oct 5 Build 64 bit

I have attached a GMW that has a meridian line over the North Pole and another one that stops at the North Pole. In the first case the line is deflected at two points despite only having only 2 vertices. I have labelled it with ???. It could be caused by the Santa Claus Effect but I am starting to have my doubts now. :). The length of the line is very close to being correct. The other line also labelled with ???, plots correctly but is off by nearly 6 km. All other 6 lines appear to be, for the most part, correct.

Note: There are two layers with exactly the same data. In the green layer the source data is geographic (produced bent line) and in the orange layer the source data is gnomonic (displays correctly). The workspace is set to gnomonic when you open it as geographic is not possible for obvious reasons. The distance measurements are the same in both layers. That is both are wrong for the second line.

I realize this is V13 but I suspect the same error will exist in V14. Any insight would be helpful. Thanks.

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2012
    Santa has imposed a "no fly" zone for the next couple of weeks, so any lines going over that region are re-routed :)

    Actually what is happening is that by default potentially ATM-crossing and projection anti-meridian crossing lines originally in lat/lon are cropped to prevent strange crossing effects. So your line is actually cropped to a lat/lon box through -180 and +180, as well as to 180 degrees from your central meridian, or +90 in this case. Then those cropped lat/lon points along the line are then reprojected and drawn separately. This allows features that cross an anti-meridian to draw correctly regardless of where the projection boundaries may be. The correct path between 2 points is really somewhat subjective anyway, in regards to whether the correct path is a straight line only in the original projection of the data points (in which case this line should kind of curve around the pole) or a straight line in whatever your current view projection is.

    You can turn this behavior off by unchecking the option in the Vector Display tab of the Configuration dialog to 'Automatically Wrap Lat/Lon Values to (-180, +180)'.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • JakeJake Global Mapper User Posts: 276Trusted User
    edited December 2012
    :o Thanks Mike. I completely forgot about the No-fly zone and trying to disable the automatically wrapping option. Cheers and have a Merry Christmas. Jake of Great White North.
  • JakeJake Global Mapper User Posts: 276Trusted User
    edited December 2012
    I can confirm the setting corrected the deflected line but the distance calculation for the other line I mentioned is still off by about 5000 m. Global Mapper gives 887577.33 m but the correct answer is: 893493.37 m.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2012
    How did you compute the correct distance? Normally Global Mapper uses the Vincenty formula for calculating distance along the great circle, but that formula is inaccurate when one or both end points is near a pole. In this case one of your points ends on the pole, so a simpler formula is used that doesn't have problems at the pole.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • JakeJake Global Mapper User Posts: 276Trusted User
    edited December 2012
    I used 5 different pieces of software and they all produced the same result. So I concluded they were correct and Global Mapper has a bug. Also, the only limitation in Vincenty's formula is for anti-podal points (which this is not). Near the pole there is no loss of accuracy. As can be seen the 5 other implementations.

    http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html

    http://www.geod.nrcan.gc.ca/apps/indir/inverse_e.php

    http://aeronav.faa.gov/index.asp?xml=aeronav/applications/programs/compsys

    http://www.defensie.nl/english/navy/hydrographic_service/nautical_applications/pctrans/

    http://www.movable-type.co.uk/scripts/latlong-vincenty.html
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2012
    I did more digging here and found that our Vincenty gives very bad results if either point is within a few meters of the pole and not just anti-podal points. I updated out calculation to chop any line segment that ends at a pole into segments so the Vincenty calculation can be used for all of the line except the last few meters at the pole, and now the results agree with what you were seeing. I have placed a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper14.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v14.xx installation folder to give it a try. If you are using the 64-bit v14 version there is a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper14_64bit.zip .

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/products/global-mapper.php
  • JakeJake Global Mapper User Posts: 276Trusted User
    edited December 2012
    Thanks Mike. I will need to upgrade to V14. Most likely after the holidays. If I get a chance before then I will test in trial mode.
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