Viewshed angular limits GM v13

KGJamesKGJames Global Mapper UserPosts: 70Trusted User
edited October 2011 in Bug Report
Hi Mike,

Here's an interesting one for you.
With projection set to British Grid, OSGB36, I tried to draw a viewshed from 335 degrees for 15 degrees. What I got was one from 340 degrees to 350 degrees, i.e. only 10 degrees wide, starting at the wrong bearing. So I tried again, with projection set to Geographic, WGS84. This time I got the full 15 degrees, but starting at 327.5 degrees and ending at 342.5 degrees. (All results angles are approximate).

I attach a text file with both sets of metadata, and a jpeg showing the two viewsheds with background map & height data hidden.

On a more trivial note, I also noticed, when setting up the WGS84 draw, that the Y-sample spacing window showed 9.something degrees, as the "e-05" at the end of the number was too far right to be in the window!

Regards
Ken

Comments

  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    Mike,
    The same problem occurs with GM v12.02.
    The workaround is for me to use wider angular limits, but having now switched to a much higher resolution height grid, run times will rise significantly.
    Ken
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2011
    Ken,

    The issue is that the angle specified is the Cartesian angle in your current projection system, not the cartographic bearing, so the reported cartographic bearing may not agree with the calculated Cartesian angle. The cartographic bearing could be used, but it is quite a bit slower to calculate that for each sample point so the simple Cartesian angle is done. If you are working in a projection system without much distortion for your area they should be close. In something like Geographic they could be quite different.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    Mike,
    At the transmitter location, British Grid north is just 2.13 degrees counter-clockwise from True north. So I still don't understand why, when specifying the viewshed angles in British Grid, I get a sector which is the right angular extent (15 degrees) but displaced 7.5 degrees counterclockwise from the start angle, while in Geographic, it's only 10 degrees wide (5 degrees less than requested) but displaced 5 degrees from my start angle.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2011
    Ken,

    Can you send a workspace with your view shed saved? I just did some tests in UTM and used grid bearings in the Measure Tool and got exactly what I specified (335 and 15 degree spread). I switched to True North and the bearings themselves changed, but I still had 15 degrees of spread.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    Here's the zipped workspace file. Mike
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2011
    I tried the file and in both OSGB and Geo if I switch my bearing measurement to grid bearing from True North or Magnetic North then I get the range 335 to 350 as expected. This is what I would expect since that is how the bearing are calculated internally (i.e. grid north vs. True North).

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    Sorry, but now I'm totally confused, Mike!
    How do I switch bearing measurements between grid & true? When I switch projections the measurement tool gives me the same bearings to within 1 arc minute.
    Where does magnetic north come into things?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2011
    If you right-click in the Measure Tool you can choose whether the bearings are reported relative to Grid North, True North, or Magnetic North at the bottom of the sub-menu. Grid North is what the view shed tool always uses.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    When I right click on the Measure Tool, no sub-menu appears!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2011
    That's odd, you should get a lengthy sub-menu. Just to be sure, you have the Measure Tool (the ruler on the toolbar) active and then you right-click on the map somewhere, correct? That should bring up a big menu with a lot of unit options as well as options for the bearings.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited October 2011
    Thanks, Mike - I didn't know that feature existed: bearing disply was set to magnetic!
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