Lab Exercise GEOG 361 651 Lab Exercise 9 Analyzing LIDAR data Dr Andrew Klein This lab was developed by Dr Klein with help from Songgang Gu and Dr Hongxing Liu I Introduction Light Detection and Ranging LIDAR is being increasingly used to generate high resolution and highly accurate Digital Elevation Models DEMs worldwide These DEMs can have spatial resolutions in the horizontal and vertical on the order of decimeters Both government agencies and commercial companies are exploiting this technology to produce DEMs for coastal areas municipalities and other areas of interest Because of the high resolution most LIDAR DEM studies are by necessity quite local In this lab you will gain experience using LIDAR DEMs acquired for the Hut Point Peninsula in Antarctica and a little closer to home in Houston Texas which has been using LIDAR to improve DEMs to aid flood planning and dealing with Houston s subsidence problem In this lab you will learn a little about the following LIDAR topics Some problems associated with minimally processed LIDAR Data Orthometric versus Geoid Heights Comparing LIDAR DEMS to more conventional DEM sources Visualizing LIDAR data in conjunction with aerial photography II Instructions A Hut Point Peninsula Ross Island Antarctica The LIDAR Digital elevation model DEM that you will be using of McMurdo Station Antarctica were generated from data obtained by NASA s Airborne Topographic Mapper ATM laser altimetry system during an extensive survey of several areas in Antarctica in December 2001 The data is publicly available from United States Antarctic Resource Center USARC at http usarc usgs gov lidar dload shtml A detailed description of the data and a thorough accuracy assessment is also available online at http usarc usgs gov lidar lidar pdfs Site reports v5 pdf A copy has been placed in the mcm subdirectory Minimal preprocessing of the original data has been undertaken and has included downloading of version 5 of the DEMs these are considered the final versions from the USARC website reprojecting the DEM Transformed projection into UTM Zone 58S in ENVI and clipping the elevation data to a coastline in ArcGIS As the original DEM covered a much larger area a subset has been selected for this class The DEM lidar dem bsq along with a Quickbird image of McMurdo Station acquired on January 1st 2003 qb 2003 01 01 hsi rect tif are available in the mcm subdirectory of the lab folder for your use Please note the Quickbird image is 200MB so please do not copy it to your home directory 1 Task 1 Please answer the following questions about the McMurdo DEM Some image processing will be required to answer the following questions Question 1 What is the spatial resolution of the LIDAR DEM for McMurdo Station Question 2 Visually inspect the McMurdo LIDAR DEM and use the Cursor Location Value Tool and Compute Statistics to examine the actual elevation values Do you notice any potential problems with the data in its raw form If so what are the problems and why might these errors exist in the original data set Question 3 What do the abnormally low elevations represent and why is this value used Question 4 How might you fix the problems with bad elevations Use ENVI to fix these bad elevation values Tips 1 ENVI has a nice little utility that will replace bad values in Digital Elevation Models Topographic Replace Bad Values The problem with the tool is that the technique used is poorly documented It is never a good idea in remote sensing to apply a function you are not 100 sure of what it is doing 2 You can also edit the ENVI header File Edit ENVI Header to allow ENVI to ignore certain data values These values are often called null values To set a certain DN value as a null data value which ENVI will ignore in its processing go under File Edit ENVI Header and then select Edit Attributes to set the Data Ignore Value Question 5 Once you have repaired the bad data values and instructed ENVI to ignore certain elevations please recompute the statistics Do these elevation values seem correct for an island why or why not You might also wish to use the Cursor Location Value tool to examine individual elevations to see if they make sense The reason that these elevations seem incorrect is that they are Geoid Heights As such these elevations are relative to the local Geoid http en wikipedia org wiki Geoid which for this dataset is the WGS84 Geoid These elevations are not wrong we simply are more typically use Orthometric Heights which is the height above mean sea level MSL However in GPS and LIDAR Geoid heights are commonly used Fortunately it is possible to convert between the two using the following equation H h N 1 Where H is the Orthometric Height h is GPS or LIDAR height measured relative to the WGS84 Ellipsoid Height and N is the WGS84 Geoid Height It is possible to use an online tool to determine the Geoid Height N and use this to create a DEM with orthometric heights 2 For the purpose of this lab it is possible to use the National Geospatial Intelligence Agency s online WGS 84 Geoid Calculator to calculate the height of the Geoid at McMurdo station which is located at 77 50 36 South Latitude 155 39 32 East Longitude The calculator can be found here http earth info nga mil GandG wgs84 gravitymod wgs84 180 intptW html In case the website is not operational the Geoid Height I determined was 52 98 m but you probably do not wish to trust me Question 6 Using the above equation and your estimate of the Geoid Height use Band Math to convert the LIDAR DEM to Orthometric Heights Question 7 Recompute the statistics for the corrected image and use the Cursor Location Value tool to see how close your new elevations are to what you might expect You may wish to use the Quickbird image of McMurdo Station to find locations near the coast to see if your elevations are believable All the white area along the southern margin of the island is sea ice Have you corrected the elevations of this DEM to your satisfaction Why or Why Not Question 8 Now for some fun use your corrected DEM and the Quickbird image in the 3 D surface view function to create a nice 3 D perspective view of McMurdo Station Include a hardcopy version of this image in your report 3 B Houston Texas LIDAR Data You will be fortunate enough to be working with a small portion of a high resolution LIDAR dataset that has been acquired for Harris County TX Working with the Federal Emergency Management Agency FEMA the Harris County Flood Control