Global Mapper v25.0

How to open Sandwell img file

a.chamoli
a.chamoli Global Mapper User
edited November 2008 in Technical Support
I am not able to open Sandwell & Smith satellite gravity data using Global mapper. Is it possible?

This is the information with the downloaded img file.

grav.8.1.img A 12672 by 21600 grid of 2-byte integers = 547,430,400 bytes.
Byte order is big_endian.
The gravity anomaly units are 0.1 milligal. An even value signifies the cell does not have an altimeter measurement while an odd value signifies that it does.
The Mercator projected image spans longitudes from 0 E to 360 E and latitudes from 72.006 N to -72.006 N.
A spherical earth is used for the Mercator projection.
The center of the upper left grid cell (i.e. the first integer in the file) is located at 72.0009 N, .00833333 E.
Longtiudes increase with a 1/30 degree spacing.
The center of the last integer in the file is located at -72.0009 N, 359.933 E.

Please help.

Comments

  • global_mapper
    global_mapper Administrator
    edited November 2008
    You just need to create a .hdr file to go along with your data file that looks something like the following:

    BYTEORDER M
    LAYOUT BIL
    NROWS 12672
    NCOLS 21600
    NBANDS 1
    NBITS 16
    ULXMAP 0.008333333
    ULYMAP 72.0009
    XDIM 0.03333333333
    YDIM 0.03333333333

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • a.chamoli
    a.chamoli Global Mapper User
    edited November 2008
    I am not able to select the overlay type while opening. If I select some format it is saying 'unable to load header'. Although I have created a .hdr file in same folder as you mentioned, but it is not working. The data set can be downloaded at:
    ftp://topex.ucsd.edu/pub/global_grav_1min/
    Can u send me the steps How to open it?
    Waiting for the reply.
  • global_mapper
    global_mapper Administrator
    edited November 2008
    I went to the site and discovered that there is already a .ers file provided for the larger image file. If you just download that .ers file and load it, it should pull your grid right in (it is still downloading so I haven't yet verified 100% that this works).

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • a.chamoli
    a.chamoli Global Mapper User
    edited November 2008
    Thanks for your words.
    I have tried with .ers file. It is opening & giving the legend in meters which should be mgal. I feel meter is default. Also color of legend is not much variable. How can I change the legend?
    I am also not sure that it has plotted it properly.
    Please open it when you complete downloading & send me your comments as I am not confident to it.
    Also, I am having some ascii data in txt format which is the region of interest in this sandwell data. The data is in three column Lat, Long & gravity value. When I am opening it the area is coming in display but there is no data.


    Regards,
    Ashutosh
  • global_mapper
    global_mapper Administrator
    edited November 2008
    Ashutosh,

    Global Mapper only supports display of gridded files as if they contained elevation values, so you'll just have to mentally note that what is called meters is really mgal.

    You can create a custom shader on the shader dropdown on the toolbar to color the data however you want based on the values (again call mgals meters) in the file.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • a.chamoli
    a.chamoli Global Mapper User
    edited November 2008
    Thanks.

    I am trying with your suggestions. Will contact you if there will be any further doubts.

    Regards,

    Ashutosh
  • a.chamoli
    a.chamoli Global Mapper User
    edited November 2008
    Hi Mike,

    I have opened the .ers file of sandwell gravity data. It is opening, but the legend & data are not properly coming. While seeing the path profile along any section the shape of data/profile is ok but the range (amplitude) is absurd. How to correct this?

    Regards,
    Ashutosh
  • global_mapper
    global_mapper Administrator
    edited November 2008
    Ashutosh,

    Your problem is likely the projection. Try selecting Mercator as the projection for the file when loading it to get the proper scaling.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com