Lidar Based nDSM Creation¶
Lab Materials¶
Download this: LIDAR Data
Lab Content¶
Objective¶
Students will create a Digital Elevation Model (DEM), Digital Surface Model (DSM), and with those a Normalized Digital Surface Model (nDSM) from LiDAR data.
Procedure¶
Steps to Create a DEM¶
Open the QTModeler Software
Import > Import Model Data
Choose LAS and QTT
Before we begin any project it is a good idea to check out the data we are working with to make sure everything is in order. We can see the metadata by clicking on a file and looking in the window at the bottom. The key detail that we are looking for that we will need later is the Estimated Grid Sampling Distance(Estimated GSD). This value is very important in determining how our data is processed. Be sure to look at each file and take note of the GSD values for each file.
Select all of your files (Hold Ctrl and click each file) and click Open
Choose Process as Group (This will merge your selected files into one model)
Enter in an appropriate Grid Sampling (Pixel Size) into the window. To find the best value we refer back to the metadata which we looked at earlier. Since we are processing as a group we want to make sure that we choose at least the greatest GSD of the files and then round up some to make sure we eliminate any error; I choose 1 for good measure.
OPTIONAL - Select Import Intensity radio button
Click the Classification button
Choose CLASS 2 Ground (assuming your data has class 2 as their ground class). This is a filter.
Make sure your settings look like the following photo then click OK and then GO.
Saving and Exporting Models¶
Once created, go to File > Save and save the DEM It is a good idea to save to your GIS Student folder which has more storage space as these files can become very large large
If you want to save your model as an item that you can open up in another program you will need to export it as a GEOTIFF(32-bit DEM)
To do this click on the Export tab and then Export Model(s)...
Make sure the window looks like the following image and then click the export button
Now you have to name your file and choose where to save it. Try to give it a meaningful name like UNG_Area_DEM.tif Be sure to not use spaces or special characters for best results
Once saved, go to File > Clear All Models or click the Trash-Can icon
Follow steps below to now create the DSM
Steps to Create a DSM¶
Import > Import Model Data
Choose LAS to QTT
Select your files
Choose “Process as Group” (This will merge your selected files into one model)
Enter in an appropriate Grid Sampling (Pixel Size), I choose 1 meter
OPTIONAL - Select “Import Intensity”
OPTIONAL - Leave “Desired Return” on “ALL” or set to “1” (1 = First Return DSM)
Make sure your settings look like the following photo then hit GO.
Save the DSM model like you did for the DEM for future use if necessary
Steps to Create a nDSM¶
Go to Edit > Subtract Model
Choose the DEM that was saved in step 11 above. You may receive a Memory Error while preforming this step. The error message might not be correct so click OK on the error message and see if it will allow you to bypass it.
Move your cursor over the image and look at the information bar below the image. You will notice the coordinates, including the Z coordinate, of your cursor is located on the bar.
Hover over on a building or tree and look at the coordinates. The number next to ‘Z’ is the height at that particular point. If you get a number that is three digits then the nDSM creation did not work. The number is in meters so if it’s rather exaggerated then that is another clue it did not work.
- The new nDSM model (DSM – DEM = nDSM)
- Note that all ground height is the same color
- Note that elevation values/coloring is now relative to height above ground rather than z value
Save your model
Analysis¶
- What is the height of the corner of the Science building indicated in the image below in the nDSM? Is this height accurate? Make sure to record these values and discuss them in the analysis section of your report.
Note: If you happened to turn the image and do not know which way is North you can click on: Display–>Options–>Show Axes
Is the large tree between the Science building the Student Center visible in the nDSM? If so, what is the height of the tree? Is this height accurate? Make sure to record these values and discuss them in the analysis section of your report.
Find the height of the tree yourself by using simple geometry and a ruler or tape measure. Make sure to draw a diagram and show calculations to include in your report. To find out how to do this there are many helpful links on the web as well as the following link: http://en.wikipedia.org/wiki/Tree_height_measurement
