| 08 Aug 2016 09:57 AM |
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Hello,
I've been working with the freely-available LiDAR DEM for the UK (UK LiDAR DEM) which uses the British National Grid projection (EPSG:27700) and is available in laz format (compressed laz files).
I've found one issue with it:
If I open the laz file, and in the Convert Format window set the input projection system to EPSG:27700, the elevations are different from when I click on Advanced and select the British National Grid projection. Although both projections are supposed to be the same thing. I suspected that EPSG:27700 in ENVI LiDAR uses feets instead of meters but it was not the case!
Anybody knows what's causing the differences in the elevations between EPSG:27700 and the British National Grid?
Thanks.
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MariM
Veteran Member
Posts:2396  
| 09 Aug 2016 10:51 AM |
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The link to the data website only appears to have .asc files and I am not able to find any .las/.laz files. Could you provide a link to a sample dataset that exhibits the behavior you describe? Also, what version of ENVI Lidar are you using?
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Deleted User
New Member
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| 09 Aug 2016 04:48 PM |
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Thanks for your response.
I'm working on the point cloud laz for the city of Tewkesbury. If you search for Tewkesbury and then look for tile SO83, you'll find the point cloud in laz format, which was acquired in 2014 (1m resolution).
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Deleted User
New Member
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| 09 Aug 2016 04:54 PM |
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Sorry I forgot to give the version ENVI LiDAR. It's version 5.3.0
Regarding the inconsistency in the height according to the projection: if you take the white building (see image below) in the tile SO83 (file SO8832_P_9938_20141212_20141212.laz to be more precise), you'll notice different values in its height depending on whether you set the projection to EPSG:27700 or to the British National Grid when you open it.
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MariM
Veteran Member
Posts:2396
| 10 Aug 2016 01:55 PM |
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I can reproduce what you are describing. The projection strings are slightly smaller but otherwise, I can't find any issue between the projections. However, the elevation values are different. I am looking into this and will let you know what I find out.
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Deleted User
New Member
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| 23 Aug 2016 09:39 AM |
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Mari Minari,
I contacted the distributor of the LiDAR data, and I've been told that the inconsistency in the elevation exists only in ENVI LiDAR (not with other softwares like QGIS, Terrascan and Arcmap), which might indicate that something is wrong in ENVI LiDAR (maybe an option that needs to be adjusted).
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MariM
Veteran Member
Posts:2396
| 25 Aug 2016 02:06 PM |
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I agree that something is not right with the discrepancy between the two projections (although they should be the same). Unfortunately, the developer has not had time to look at the problem yet due to other deadlines but I will ask again to have a look to confirm if there is a bug.
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