After wrestling with issues relating to geotiff images –
and receiving several good responses, I will summarize what I have discovered
about geotiff and rotated images in MapInfo.
- I was having difficulty with
geotiff images coming in with the wrong projection information. Rich
Greenwood suggested a open-source raster editor called gdal tools (http://www.remotesensing.org/gdal/)that
allowed me to view the headers on the geotiff files. What I found
out from this was that many of the Geotiff files had the wrong projection
information in them (We use State Plane – feet coordinate systems
for most of our work – and we routinely have difficulty with
software that only recognizes State Plane – Meter coordinate
systems).
- MapInfo WILL read the
rotation parameters from tif–tfw, and geotiff images and create
rotated coordinate systems for them. As with all raster images,
MapInfo creates coordinate system on the fly that it will use to warp vector
layers to when drawing Map windows with these images. A rotated tif
image is no different – except that north isn’t up.
- There may be some problems with
the accuracy of the coordinate systems on rotated geo-tif images. I
tested the rotated and unrotated images I had with by overlaying a survey that
had been done on the site. Over a distance of 23,000 ft. from the
upper left corner of the image – the location of vectors relative to
the photo had drifted ~20 feet on the rotated image. This could have
been due to all of the other projection issues I had with this file, but as
with all raster images registered in MapInfo – location should never
be taken as absolute without a good understanding of the accuracy or the
geo-registering, or orthorectification process.
- There are times when it is
helpful to be able to rotate an image/coordinate system at a site because
of local grid systems that are used for exploration and/or mining.
- There is probably a problem
with the geotiff header files written by ENVI (this was suggested in a
previous discussion of this rotation issue). When looking at the
header files for the Geotiff image from ENVI the “geotransform”
parameters for Northings appears to be reversed (or have the wrong
sign). Using the gdal tools mentioned above, I was able to create
a Virtual Raster file (VRT) from my ENVI geotiff – edit the geotransform
parameters, and translate it again back into a geotiff – and then
bring it into MapInfo such that the coordinate system warped the vectors
accurately (again Rich Greenwood was very helpful with this). I
have contacted ENVI about this to see if they can investigate how their
program creates geotiff files – and hopefully to correct it.
- My first question – how to
dumb-down a geotiff?? Open it in Photoshop – and save it…(doh!)
Thanks again for all of the responses to this question.
Ann Moulding
GIS Specialist, Geologic Mine Services
TXI Operations
Office: (972)-647-3827
Cell: (214)-502-0551
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