Surface area vs Fill surface area

seb3343seb3343 Global Mapper UserPosts: 31Trusted User
edited March 2014 in Elevation Data
Hello-
I have a question regarding the difference between the surface area (obtained using the feature info tool) and the fill surface area (using the measure volume tool). I my case, I have a bathymetric map (i.e., all "elevation" values are negative) and an area feature on top of it. When I click on the feature information tool, I see a value for the surface area (let's say 10000 sq m) and the average elevation (7m). If I click on the volume tool, I obtain a fill surface area that is ~1.8x the surface area (2d and 3d fill surface areas are the same and cut surface area is 0). I was expecting to obtain the same value. How can these values be so different? I double-checked that I was using the same units, and that the sample spacing was low enough.
Thank you!

Comments

  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    Additional information that leaves me totally puzzled. I tried with Global Mapper 13 on 2 different computers, and with Global Mapper 14 on one computer.

    When I draw a line of approx 1 km along the scale bar, its "length" as seen with the info tool is only 770 m. Similarly, if I draw a rectangle of 1x1 km (according to the scale bar) the surface area seen with the info tool is 0.54 sq km. If I use the measure volume tool, the surface area is correct (1 sq km).

    I noticed this problem when using the Mercator projection. If I use a simple geographic projection (lat/long), the surface area calculated using the 2 different methods is the same. However, when I draw a line of 1 km along the scale bar, it's measured length is still 770m ...
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2014
    What you are seeing is the result of severe distortion between the linear units of some projected systems (Mercator is particularly bad) and ground truth on the ellipsoid. If you right-click in the Measure Tool and specify to use grid distance rather than great circle then your distance based on the scale bar should match the reported distance, but will be way off from ground truth if you are using Mercator far from the equator where everything is stretched longitudinally.

    The volume calculation breaks everything up into tiny squares and computes the area of one of those assuming the projection units are close to ground truth (i.e. it assumes 1 meter in projected Mercator meters equals 1 meter on the ground), which is close to true for most projections in a small area. If you switch your projection to something like UTM that should make the volume results much more closely match the Feature Info Tool area.

    For the latest build I have also updated the volume calculations to automatically detect highly distorted projections and use a smarter area calculation for the sample cells so that you should automatically get nearly identical (there will be slight difference due to boxes used vs. polygons) results even with very distorted projections like Mercator. I have placed a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v15.xx installation folder to give it a try. If you are using the 64-bit v15 version there is a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15_64bit.zip .

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    Thank you very much for your answer, Mike.

    Thanks for updating the volume calculations in the latest build. I am using v 14 at the moment (haven't bought the v15 upgrade yet) so I guess I can't try the new build at this time.

    Regarding the scale bar, I agree that if I
    specify to use grid distance rather than great circle, the measure I get using the measuring tool matches what is represented by the scale bar. However, all the measurements become "incorrect", i.e., about 40% too long compared to Google Earth or to regular topographic maps. What I would like to do is change the scale bar to use the "great circle" distance, i.e. to have the scale bar match what I see in GE or on topographic maps. Is it possible? Maybe I should just use UTM projection instead of Mercator?

    Thanks again!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2014
    The problem with the scale bar is that the X and Y scales are vastly different in Mercator, so if you drew a line the scale bar length from left-to-right and compared it's great circle distance to one of the same length on the screen but drawn top-to-bottom you would get significantly different values, so there can't be a single scale bar that is accurate no matter where you measure on the screen. If you switch to UTM the X and Y scales for any location in the zone should be close enough that you likely won't notice, but for a distorted projection like Mercator or Geographic (lat/lon) you simply can't have a scale bar that is accurate in all directions.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    Great, thank you for the clear explanation. I will switch to UTM then.
  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    Dear Mike,

    I have been thinking about this issue a bit further, especially about your statement that "the X and Y scales are vastly different in Mercator". There are 2 mains reasons for us to use the Mercator projection: (1) the geographic coordinates (lat/long) are perpendicular (in other words the different parallels are always represented parallel to each other), and (2) the linear scale is equal in all directions (from wikipedia). Since we only use it for small regions (a few 100 km max) at the mid latitudes of the southern hemisphere, the distortion is not important. I agree it would be important if we were comparing objects close to the equator to objects close to the poles.

    When I measure an object in global mapper using the measuring tool and a Mercator projection, the x and y scales are equal. The x and y distances measured with both the "great circle" and "grid distance" are equal (but the "grid" distance is always much higher than the
    "great circle" distance). The issue is that the distance indicated by the scale bar always matches the "grid" distance. We would like to make it match the "great circle" distance, which is much closer to the "real" distance (as measure on UTM maps, directly in the field, in google earth, etc). Is this possible?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2014
    I have decided to update the scale bar display for "highly distorted" projections like Mercator where the scale varies wildly from place-to-place to actually calculate the average of the width and height in meters of a single pixel at the center of the screen. For Mercator this makes the scale bar much closer in distance to the great circle distance, although of course it won't be perfect, especially when zoomed out or when using projections where the X and Y scale don't match.

    I have placed a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v15.xx installation folder to give it a try. If you are using the 64-bit v15 version there is a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15_64bit.zip .

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    Perfect, thank you very much. I haven't been able to try it yet since I do not currently have version 15 but according to your description, this should fix the issue. Thanks again, Mike.
  • seb3343seb3343 Global Mapper User Posts: 31Trusted User
    edited March 2014
    I have just upgraded my version of Global Mapper to v15 and I uploaded the new build. I can confirm that the two issues discussed above were fixed. Thanks a lot!
  • Ice Age MarkIce Age Mark Global Mapper User Posts: 299Trusted User
    edited March 2014
    Howdy Mike,

    Don't know if it's related to this change, but since then: When I use Shift+z and enter a scale to zoom to, GM does not go to that exact scale, but rather to one that is only somewhat close; e.g. (1:)24,000 becomes 1:23,970. Doesn't seem right, although the change itself seems like a good idea. How would I get to the exact scale that I want?

    Thanks for looking into this,

    Mark
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2014
    Mark,

    Are you using Mercator for your display projection? The change to improve the volume calculations in highly distorted projections like Mercator did affect going to exactly a particular scale in those projections as there really isn't such a thing as an exact match to the scale in multiple directions. Before it would use the units of the projection to calculate the scale, which was good enough for most projections and could match exactly, but for a high distorted projection system like Mercator the reported scale was way off from ground truth.

    In the latest versions it should be much closer to ground truth for Mercator, but that also makes it harder to get exactly what was requested. It should affect a projection system that isn't heavily distorted though, only Mercator projections.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • Ice Age MarkIce Age Mark Global Mapper User Posts: 299Trusted User
    edited March 2014
    Hello there Mike,

    So it is. I am in fact using Transverse Mercator with a central meridian of -108 because I want a nearly vertical meridian at the center of my view. With Orthographic projection the scale becomes the exact entered value using Shift+z; with TM and UTM it does not.

    I would prefer that volume calculations be more accurate if it's a choice.

    Thanks for explaining, I'm getting old and get confused.

    Mark
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2014
    Mark,

    Ah actually the change was only supposed to affect the standard Mercator projection and not also Transverse Mercator and UTM, but apparently it was hitting them too. I've fixed that so now the scale setting works as expected for UTM and TM. The volume should still be correct for all. I have placed a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v15.xx installation folder to give it a try. If you are using the 64-bit v15 version there is a new build at http://www.bluemarblegeo.com/downloads/global-mapper/global_mapper15_64bit.zip .

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • Ice Age MarkIce Age Mark Global Mapper User Posts: 299Trusted User
    edited March 2014
    Works just fine now. Thanks very much Mike.
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