Digitiser - Point at specified location

KGJamesKGJames Global Mapper UserPosts: 70Trusted User
edited February 2009 in Suggestion Box
It would be nice if the Lat/Long input option allowed the correct datum to be chosen.

Thanks!

Ken James

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2009
    Ken,

    You setup the projection/datum to use on the Projection tab of the Configuration dialog.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    Mike,

    Now I'm really confused!

    Attached is a workspace file to illustrate my problem. I loaded 4 text files, WGS84 and ETRS89 are identical, and define a Lat/Long 0.1 degree "square" with bottom left hand corner at 51N, 1E. Loading one with WGS84 as datum, the other with ETRS89, and they're obviously identical. So far, so good.

    The other 2 files, OS-ETRS and OS-OSGB are also identical, with Eastings & Northings equivalent coordinates, as converted using the Ordnance Survey's "definitive" tool as found at : National GPS Network - Coordinate transformer

    They were loaded as British Grid, datums ETRS89 and OSGB36 respectively. The ETRS89 load doesn't match up with the "square" defined by Lat/Long, but the OSGB36 data does. So far, so good.

    Now set the projection to British Grid, ETRS89, and turn on a Lat/Long grid, and close in on one of the corner points. The grid and the input data (Lat/Long & E-N OSGB) line up perfectly, the Lat/Long cursor readout is perfect, but the British Grid coordinates are 80 to 100 metres out from the input data.

    Now change the projection to British Grid, OSGB36. Oops! The Lat/Long grid moves away from my "square" - but my input Lat/Long and eastings & northings stay together. Now the cursor readout of the British Grid coordinates is correct, but the Lat/Long readouts are not.

    Is this just a bad dream, or am I doing something stupid?

    Thanks, in hope!
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    P.S.
    Global Mapper's coordinate converter, for ETRS89 Geodetics to/from British Grid/OSGB36 cartesians, produces results that are identical to those from the Ordnance Survey's tool - well, within a centimeter or better!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2009
    Are you sure you aren't reversing which data set is ETRS89 and which is OSGB36 when you are loading them? From the behavior it sounds like you may have them flipped. Also keep in mind that a given lat/lon value is NOT at the same location on the earth in ETRS89 and OSGB36 (it wasn't clear to me if you understood this as it is not common knowledge unless you are familiar with datums).

    For example, an ETRS89 coordinate of 1W 46N corresponds to an OSGB36 lat/lon of 0° 59' 55.2383" W 45° 59' 55.9585" N. Also keep in mind that the lat/lon grid displayed in Global Mapper is a lat/lon based on the current datum selected on the Projection tab of the Configuration dialog, so if you change your datum there your lat/lon ground will likely shift to a different location relative to the loaded data.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    The numerical values in the pair of Lat/long files are identical. Same goes for the two Eastings/Northings files. I duplicated them & gave different names only so I could remember how I loaded them.
    I haven't used any OSGB36 lat/long coordinates, only WGS84 & ETRS89 (which are identical). As I indicated, my OSGB Eastings and Northings were derived from ETRS89 Lat/Long by using the Ordnance Survey's official tool, and Global Mapper's coordinate converter gives exactly the same conversion results.
    The same points, as defined in Lat/Long and Eastings/Northings, are overlaid exactly on the display, so Global Mapper's input conversions match them perfectly. Why can't I get a cursor readout (British Grid plus Lat/Long) that simultaneously matches my two forms of input for the same point?

    Thanks!
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    And PS

    1 W, 46 N is way down in France, well outside the validity limits of any OSGB transformations!

    Thanks
    Ken
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2009
    Ken,

    I just picked a point off the type of my head as an example that I thought was near the UK, I didn't check a map. The point is the same though.

    In any case, the coordinate readouts on the status bar will always be in the datum selected on the Projection tab of the Configuration dialog. It sounds like your lat/lon values are in WGS84 (or ETRS89, which is basically identical), while your easting/northing values are in British Grid with the OSGB36 datum. Since you can only display coordinates in one datum at a time, you will not be able to get a coordinate readout on the status bar that reflects your lat/lon values in WGS84 and your easting/northing values in OSGB36 since those datums are different. You can only have one or the other at a time.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    Mike,

    Right, I think I'm getting there!

    If I understand you correctly, when my chosen projection is British Grid & OSGB36, the cursor readout gives me the correct OSGB36 Eastings and Northings as cartesians, and Lat/Long on the OSGB36 spheroid.

    Here in the UK, we use OSGB36 Eastings and Northings as cartesians, but ETRS89 (=WGS84) Lat/Long for the geodetics. That's why the Ordnance Survey definitive coordinate converter tools only convert between these two coordinate systems. As far as I know, we in the UK don't use OSGB36 Lat/Long any more.

    Thanks!

    Ken

    So can I change my suggestion to:

    Can we please have an option to pair WGS84 geodetics with British Grid/OSGB36 cartesians, for cursor position readout, gridline display, and defining points at specific locations?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2009
    Ken,

    I have added a new option to the Advanced section on the General tab of the Configuration dialog to specify that lat/lon values in the status should be displayed as WGS84. I have placed a new build at http://www.globalmapper.com/global_mapper10.zip with the change for you to try. Simply download that file and extract the contents into your existing v10.xx installation folder to give it a try.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • KGJamesKGJames Global Mapper User Posts: 70Trusted User
    edited February 2009
    Mike,

    That's great, thank you!

    I look foward to a global "Use WGS84 for all Lat/Long"

    Ken
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