Re: [LAStools] Merging tiles together without adding in diagonal noise

[Ty Kennedy-Bowdoin]

Hi Ben,

I am not entirely sure that I understand your processing stream so I apologize if my answer is off base... 

It seems to me that you dont need to output the rasters two times (lasgrid and las2dems) but maybe you not just outputting the DTM. Depending on how large your dataset is you can probably use blast2dem (http://www.cs.unc.edu/~isenburg/lastools/download/blast2dem_README.txt) to output your DTM from your las/laz tiles:

blast2dem -i *.laz -o big_dtm.tif -keep_class 2 -merged -step 1

Of course you can use bil if you prefer. 

Now sometimes, you may not be able to use blast2dem because you have too many points (~billions). In that case, you may need to output the tiles as individual rasters, then merge the rasters. I usually use las2dem much like you tried, but then I merge them using gdalbuildvrt and or gdal_translate:

http://www.gdal.org/gdalbuildvrt.html

http://www.gdal.org/gdal_translate.html

I have found that this method does have some tricky bits if you want to avoid edge artifacts. 

• You want to lastile with a -tile_buffer (I usually use 10% of tile_size).
• when you run las2dem, try to use resolutions in 1 m increments (-step) otherwise you can end up with some missing rows or columns on tile edges (black lines). I think this is related to the tile_size not being divisible by step size, leaving potential for some rounding error. In practice I stick with tilesize 1000 and step 1m when ever possible.
• using las2dem, you should use -extra_pass to filter the points before interpolating them (seeing as you are only using ground and that will remove a lot unnecessary processing).
• use tile_bb to keep overlapping tiles, otherwise you will likely see some artifacts and gaps near the tile edges because if you -use_orig_bb you will only interpolate up to the original edge of the tile without the buffer 

Hopefully that will help get you on your way. If not let me know,

Cheers,

Ty

On Thu, Jan 31, 2013 at 5:21 AM, Ben Robson <benrob...@gmail.com> wrote:

Hi,

I'm trying to create a DTM beneath a vegetated area in Eastern Norway. I've ran lasground on my tiled data, however when I try to merge the data together I get the black noisy lines added into my result. Following the advice of an earlier post, I ran las2dem outputting to BIL format, and using the tile borders, then I used lasgrid with the command -merged to try to create one TIF file. The only way I have managed to do this successfully is using ArcMap, but for completeness sake I'd like to be able to do it in LAStools. Can anyone help with this?

Thanks,

Ben

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[Newcomb, Doug]

Ben,

try gdalbuildvirt, http://www.gdal.org/gdalbuildvrt.html, and gdalwarp, http://www.gdal.org/gdalwarp.html.  Bear in mind, TIFF is only good up to 4 GB , then you have to use bigtiff format

Doug

On Mon, Feb 4, 2013 at 2:30 AM, Ben Robson <benrob...@gmail.com> wrote:

Thanks for the response Ty, so it isn't possible to merge all the tiles together using LAStool?

Ben

On Thursday, January 31, 2013 2:21:36 PM UTC+1, Ben Robson wrote:

Hi,

I'm trying to create a DTM beneath a vegetated area in Eastern Norway. I've ran lasground on my tiled data, however when I try to merge the data together I get the black noisy lines added into my result. Following the advice of an earlier post, I ran las2dem outputting to BIL format, and using the tile borders, then I used lasgrid with the command -merged to try to create one TIF file. The only way I have managed to do this successfully is using ArcMap, but for completeness sake I'd like to be able to do it in LAStools. Can anyone help with this?

Thanks,

Ben

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[Ty Kennedy-Bowdoin]

Ben,

You can certainly merge the tiles in lastools as I mentioned using blast2dem with a -merged  parameter (also a number of other tools accept a merged parameter), I only added the tiled approach in case your dataset is too large for the blast2dems tool (~several billion points). 

An add-on to Doug's comment, gdalwarp can be a bit cumbersome compared to gdal_translate. If you aren't actually warping (reprojecting) you probably dont need to run gdalwarp.

Cheers,

Ty

[Ben Robson]

Hi Doug,

Thanks for the reply, I am quite new to this sort of processing so please forgive my ignorance! So I see the example command at the bottom of gdalbuildvrt to make a virtual mosaic, how would I save the file as a bigtiff?

Thanks,

Ben

[Martin Isenburg]

Hello Ben,

because "lasgrid" can be (ab-)used as a "demmerge" tool and because lasgrid can read *.asc and *.bil and write *.tif or - for BIG outputs- *.img you could have a tile-based pipeline (see the ./lastools/example_batch_files folder) that produces BIL or ASC and then have a final

lasgrid -i *.bil -merged -mem 2000 -o dem.img

to put them together.

Regards,

Martin @rapidlasso

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http://rapidlasso.com - fast tools to merge BILs into IMGs