Global Mapper v25.0

export geotiff from USRP : one pixel move

LRAYNAL
LRAYNAL Global Mapper User
edited December 2007 in Technical Support
hello,

i would like to export a part of an USRP image in geotiff format

i have used the Export Raster and Elevation Data\Export Geotiff function

i use the draw box button to draw the part of USRP image

then the geotiff file obtained is moved from one or two pixel
from the original USRP image

have you an idea of parameters that i have not selected ?
is it a limit of global mapper ?

thanks for your help

Comments

  • STH
    STH Global Mapper User Trusted User
    edited December 2007
    Have you zoomed in far enough to see the difference? If you have a full view it may look like they are moved one or two pixels, however when you zoom in so you can see the actual pixels they may not move. Is the problem still there when zooming in?

    UPDATE: You may also try to UNCHECK the Force Square pixels option, as this may add a pixel or two in rows or columns. This may be the reason
  • LRAYNAL
    LRAYNAL Global Mapper User
    edited December 2007
    i confirm that there is a displacement even when you zoom in.

    i have make the test with UNCHECKing the Force Square pixels option
    but there is the same problem (seems to be less significant) but there is still a
    small move.

    i don't know the command that allows to see the image at pixel size.

    i have attached two snapshot of usrp only and usrp+geotiff. you will see
    that the geotiff seems to be not well placed.

    thanks
  • STH
    STH Global Mapper User Trusted User
    edited December 2007
    Well, you see it at pixelsize on the image. Each square is one pixel. It seems that the image is shifted one pixel north.

    Living in Norway I am not familiar with USRPs. But what do you use as input for the location of the tif-file? Can you post a summary of the GeoTIFF/ERS/WORLD or other coordinate system of the input file and the output file? Is the pixel sixe if the image correct?

    I try to help because I have experienced something similar myself, but then it was the Force Square-error that added the extra line.
  • global_mapper
    global_mapper Administrator
    edited December 2007
    I believe the problem is a pixel alignment/size issue. From your screenshot, it appears that your exported GeoTIFF pixels do not align perfectly with the input USRP pixels, nor would I expect them to since you drew a new arbitrary box for the bounds of the exported GeoTIFF file. As it appears that you do not have interpolation enabled, each exported pixel in the GeoTIFF file will have whatever color is in the input file(s) at the top left corner of the pixel (this is called the nearest neighbor export). With nearest neighbor and un-aligned pixels there can be what appears to be up to a one pixel shift, just depending on where the the pixel sample locations line up within the input pixels.

    I would suggest enabling interpolation for the input USRP layer prior to export. You can do this from the Overlay Control Center by selecting the layer, pressing Options, then checking the Anti-Alias box. This will then calculate an interpolated color at each sample location that is more representative of the color regardless of pixel alignment. Another option would be to try and make sure that your output export bounds are aligned on exact pixel boundaries in the input data, but this is harder to achieve.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com