QGIS 4 Arch - LiDAR to DEM with LASTools

in this tutorial we use QGIS and Elias tools to convert a lidar point cloud into a digital elevation model or de M a DM is a visual representation of the ground elevation within a certain area this is different from a digital surface model or DSM which includes all features within that area buildings trees and so forth so we'll begin by installing la s tools and activating the QGIS la s tools plugin then we'll download some lidar data inspect it and divide it the final steps involve converting the split files into several de M files before merging them into a single image or raster file representing the ground elevation of our study area as always timestamps are available in the description so you can jump from section to section ok let's get started our first step is to download la s tools and then activate this in QGIS so the link for this page is in the description below click the download link here this will open up zip files once that's completely downloaded we can then go ahead and take the la s tools folder and copy it to your local hard drive I tend to just drag and drop it onto my C Drive you can see it's already installed here our next step is going to be activating this plug-in in QGIS so go ahead and open your manage and install plugins window and of course what we're going to search for here is la s tools click on this and install the plug-in once this is complete we can click close so once we've installed the la s tools plug-in we have to make sure that it's pointing to the correct directory and we do this by accessing the options on the processing toolbox we want to expand the providers section and we can see la s tools here we want to activate it and we want to make sure it's in the correct folder since I installed it to my C Drive this is correct you could of course edit this and then select a different folder if that's the case on your machine click OK and that's it you've now installed la s tools plug-in in QGIS our next step is to download some lidar data I'm going to do that at the National map download site from the USGS but of course if you can't find data related to your area you can search for lidar data download geographic region so let's go ahead and navigate to an area I'm just gonna go ahead and navigate in here to an area I'm familiar with Fayetteville Arkansas at the University of Arkansas and we're gonna drag a selection point here actually a box and then we need to select the type of data we want to use in this case we're looking for lidar so we'll click this box we'll make sure light our point cloud is selected we can click the show availability to make sure that it is here and it is you can hide that and then click find products because I've got a relatively small area selected here this is only going to be one lidar tile so that's useful there's just one thing to download we can go ahead and preview this and then download either the LS or la z la Z is the compressed form of an LA S or lidar point cloud file format go ahead and download la Z file or la s it doesn't really matter but CL AZ it's just going to be directly to the file so I'll save this I've already done it so click we can actually overwrite that so now we have that ready to go okay now that we've downloaded our lidar point cloud we can go ahead and inspect it and we want to do this because we're gonna get some information from this that helps us with our next few processing steps and the tool we're going to use to examine our light our point cloud is called la s info and so open this tool and navigate to the la s or la Z file you just downloaded click run we don't have to worry about these other settings and once that's finished we'll see that this window gets populate with a whole lot of additional data and so there's a couple of things we want to pay attention to we can look at a whole lot of different stuff here we can see classification point codes we can see number of points we can also even see the coordinate reference system use so if we scroll up a bit we can see here the coordinate system used so this is nad 83 it's a UTM zone 15 north we can also scroll down and since this is a point cloud right the original data is massive points representing lidar return so this is an area or these are points where the laser has hit something and bounced back so that could be leaves it could be the top of buildings it could be the ground and so forth and good light our data will have been classified and we can see here we have this many points that have been classified as Brown so as we continue to create our digital elevation model or DM these are the points that we're going to be using so go ahead and close this out and we can move on to dividing this lidar data the unpaid version of la s Tools has a size limit for the files that it will process so in other words if you use a file that's larger it's going to introduce error now there's a way to get around this we can split our downloaded lidar files our la s or la Z files into smaller files LS tools actually comes with a really convenient tool for this called la s split so go ahead and open up that tool and navigate to your downloaded file I'm gonna go ahead and stick with the LA Z file click open the number of digits we're gonna reduce to 2 this is just the number of digits it appends to the end of the file name we do want to split by flight lines that's fine interval or number we'll leave it at the default and then we do need an additional command line parameter and this is - or - split and put in 1 million what this does is this gives an upper limit to the size of these split files so that we can work within the unpaid versions restrictions click run and when this is finished it'll have created a whole new set of files in that folder where the original file sits so if we open up that folder we can see these new files and here we have them it's actually converted them to la s and that's partly because this output la s la z file often doesn't work I'll use it I'll try to input something and I get an error message I leave it blank and what that means is it just dumps them into the same folder and so what I do is I create a split folder I just drag and select all of these newly created files and into that split folder so what we're going to be doing now is batch processing all of these files in our next steps so our next step is to take those individual la s files and create our digital elevation model or D en files and so we have a couple of different tools we can use for this in fact if you search for la s2 de em you're gonna see a couple pop up you can also see that there's other things you could convert this to we're going to stick with de em if you went with la s2 de em that would work on one file that la s2 de empro works on multiple files so that's the tool we're going to use this is we're putting all of those files into their own directory comes in really useful so we want to specify the input directory so go ahead and navigate to that directory select that folder we know that the previous tool created la s files so we want to update that we definitely need to change the filter to keep class 2 scroll down we do want to click run new 64-bit executable and we want to go ahead and specify an output directory and I'm just going to specify the same directory go ahead and click run and this will work through each of those ten files creating now basically different image files which we can see if we go into that folder on the hard drive will actually see these TIFF files being generated so we've now converted all of our individual la s files to corresponding DM or image or raster or TIFF files now that we've created our individual DM or raster files we have to merge them into one the easiest way to do this is use the pre-loaded merge tool that comes with QGIS we're going to choose the one under raster miscellaneous open this and navigate to the folder and select all of these files and remember what we're looking for are the TIFF files select all of those click ok everything else here is fine we want to go ahead and save this to a permanent file I'm going to go ahead and save it to merged lidar tiff and then we can go ahead and click run now we're going to see some errors here that's because there hasn't been a coordinate system assigned to this that's fine this may or may not show up for you depending on the data you've downloaded but we can fix that very quickly as you recall we already looked at the coordinate reference system when we ran the la s info tool and we know that in this case it's nad83 UTM zone 15 north so if we select that we now know that this is referenced correctly even though this doesn't really look like lidar data or elevation data but if we change this to hillshade it immediately snaps into focus and looks very clear to us now one of the things that jumps out here is obviously some of these points that were being used to create this digital elevation model right so the ground surface some of these points have been classified as ground points even though they're not obviously here in the center is a football stadium there are methods for taking out these points and I'll cover those in other videos there's our workflow for using free tools to convert a lidar point cloud into a digital elevation model so as always links to location of data are in the description make sure to hit that like button and subscribe to get future updates until next time keep mapping the past [Music]

Channel: AnthroYeti

Views: 12,299

Keywords: QGIS, Tutorials, Archaeology, LiDAR, DEM, LASTools, GIS

Id: FFLxnBTgtnw

Length: 10min 35sec (635 seconds)

Published: Wed Jul 08 2020