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I've downloaded the UK's LiDAR elevation data (see below) in laz format for a specific region (Tewkesbury) from: http://environment.data.gov.uk/ds/survey#/download

LiDAR data

From that, a DSM (Digital Surface Model) was generated using ENVI LiDAR in ENVI (hdr) format as shown below (the red cross represent the coordinates of the building of interest):

DSM image

By comparing the previous DSM image (in the British National Grid EPSG:27700 projection) to the optical image of the same area found on Google Maps (see below), I noticed that the same point appears at different locations.

Optical image

Now what I would like to do is to co-register the elevation DSM data with the optical image (or potentially a radar image), but this seems as an additional difficulty besides the differences in the resolution. How can I deal with the differences in the projection between the two images? Should I (if possible) project the DSM elevation data to the standard WGS84?

Hakim
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1 Answers1

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There is nothing wrong with your data. It is just the fact that OS still issues coordinates in OSGB36, while Google uses WGS84:

enter image description here

So you have to assign EPSG:4277 to your degree coordinates, or EPSG:27700 for raster data in projected coordinates. Make sure that both projections have a `+towgs84' datum shift.

AndreJ
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  • Which of the WGS84 and the EPSG:27700 is the more accurate? and where should I add the datum shift +towgs84? – Hakim Jun 08 '16 at 07:07
  • Both are equally accurate, but have different opinion of where the zero meridian is. The datum shift is part of the projection definition, the implementation is depending on the software you use. There are different shift datasets around, see http://gis.stackexchange.com/questions/40461/raster-incorrectly-reprojected-to-osgb27700 for an overview. – AndreJ Jun 08 '16 at 11:44
  • Can you explain in more details how to reproject the LiDAR data from EPSG:27700 to WGS84 using QGIS or ENVI...? – Hakim Jun 09 '16 at 02:31
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    In QGIS: Raster -> Projections -> Warp selecting a different filename and target CRS. – AndreJ Jun 09 '16 at 05:48