Dear Frank,
Many thanks for your e-mail and such a rapid reply!
I don't want to suggest that there is necessarily a problem with the OGR
library, it could well be that I simply don't understand what it is doing
of course.
An example file is available on the OS web-site:
http://www.ordnancesurvey.co.uk/oswebsite/products/landformprofile/sampledata/lfprdntf.exe
(self extracting zip file)
and the technical reference which describes the format at
http://www.ordnancesurvey.co.uk/oswebsite/products/landformprofile/pdf/D05300_23%20Land-Form%20PROFILE%20UG%20TS.pdf
(see pdf page 29 onward).
In the NTF file the first record of 'type' 50 (i.e. data) starts
"510000000100000501-0030-0033..."
which I think means the first two grid point have heights -3.0m and -3.3m
respectively (Z_MULT=0.1 hence X by 0.1).
In the OGRINFO -la output for the file (produced using FWtools246 under
windows XP pro within a 32 bit virtual machine) the first two features
are:
OGRFeature(DTM_SS78NE):1
HEIGHT (Integer) = -3
POINT (275000 185000 -3)
OGRFeature(DTM_SS78NE):2
HEIGHT (Integer) = -3
POINT (275000.0 185010.0 -3.299999952316284)
What I can't understand is why the height is reported as an integer and
also where the figures after the decimal point in the z coordinate are
coming from.
Overall what I am hoping to achive is a simple text file listing X, Y, Z
(one per line of the file) coordinates for the land surface which can be
fed into a surface generation program (surfer).
Any help in understanding the OGR output would be appreciated.
Best Wishes,
Alex
On Sat, 14 Nov 2009, Frank Warmerdam wrote:
Alex,
Can you provide access to an example file? If
distribution is an issue, you can make it available just
to me.
Best regards,
On Sat, Nov 14, 2009 at 1:10 PM, A.J. Carter <ajc...@maths.leeds.ac.uk> wrote:
Hello,
I was hoping someone might be able to help me with a query about using the
OGR library to read an Ordnance Survey Landform Profile file in NTf format:
http://www.ordnancesurvey.co.uk/oswebsite/products/landformprofile/techinfo.html
The file contains a 501 X 501 grid with height values at each point. From
the technical reference on the OS web page it lists this as being stored
within a field called Z_COORD. Manually inspecting the NTF file (which is
plain text) I can see values like 00159 meaning 15.9m AOD. If I run the NTF
through OGRINFO -al on the file produces a feature for this point:
OGRFeature)DTM_SD19SW):1
?HEIGHT (Integer) = 15
?POINT (310000.0 490000.0 15.899999618530273)
At the moment I can't understand why it's writing out the height as an
integer, and also where the many decimal places in the z coordinate of the
point have come from.
Any help would be much appreciated.
I'm relatively new to GIS software; sorry if this is a silly question!
Many thanks,
Alex
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