Fuels Inventory and Management
Fuels Inventory and Management FOR 451
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This 14 page Class Notes was uploaded by Ms. Alene Howell on Friday October 23, 2015. The Class Notes belongs to FOR 451 at University of Idaho taught by Staff in Fall. Since its upload, it has received 12 views. For similar materials see /class/227834/for-451-university-of-idaho in Natural Resource Ecology And Mgmt at University of Idaho.
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Date Created: 10/23/15
FOR 451 Fuels Inventory and Mapping 42 An Overview of Remote Sensing Mapping Methods II Lidar 9i University of da 0 56 b A s i J FOR 451 421 Lidar for Forest Inventory Mapping Lidar stands for gght Qetection and Banging A Lidar system is essentially a Laser Range nder that has been strapped to the belly of an airplane This laser shoots pulses of light towards the ground The time for the light to travel to and from the target is used to determine distance LASER SCANNING z r Distance Speed x time 39i This distance and the position of the airplane is used to get elevation and location In recent years aerial photography analysis has given way to lidar Lidar or Light Detection and Ranging uses laser systems to bounce pulses of laser light off targets By measuring the time taken by the laser pulse to take to travel from the source to the target and back to the sensor and using the simple relation between distance speed and time the distance between the sensor and the target can be measured These distances are used to provide information on the relative elevation of objects such as buildings or trees If a known elevation exists such as say the ground then these relative elevations can be converted into absolute heights It is though this approach that lidar has become immensely popular in the analysis of forest inventory as the absolute heights of trees within a stand can be measured Importantly Lidar is a MEASURING tool It is after all simply a laser rangefinder stuck to the belly of an aircraft Just like a laser range nder it provides a Measure of the distance and from this the tree heights This is quite different to other remote sensing methods that generally INFER information from how re ective a surface 1s FOR 451 421 The Lidar Pulse Footprint from laser pulse Each pulse of laser light contains a large number of light pulses Of those sent down by the laser system very few make it all the way back to the sensor Those that do are called returns The 1st return to make it back to the sensor might be a tree top while the last return could represent the ground IMPORTANT NOTES Wnere no tree exists the 151 could also be the LAST return The Last return might NOT be the 7 ground The lidar system shoots several laser pulses from the aircraft to the targets Of these pulses that are sent down some will be absorbed by the targets some will be re ected in random directions some will hit branches or other objects some will hit the ground Very few of the pulses sent down actually return to the sensor Those that do are given the name returns as they retumed to the sensor In forest inventory assessment we generally assume that the rst return to make it back from the targets represents the highest target while the last return is from the lowest target Now if we were say shooting the pulse at a closed canopy forest then the rst return might be due to the top of the canopy while the last return might be from a pulse that actually reached the ground Notice I used the word might Importantly the 1st return might not be from the top and the last return might not be from the ground They are from whatever object the pulses intercepted Consider a closed canopy the lSt and last return might be both from the top ofthe canopy while in a clearcut they might both be from the ground As you can imagine many other objects may also prevent the last return from hitting the ground Questions Give examples of what the last return might hit when its not the ground Snow branches houses tanksl people animals shrubs boulders etc FOR 451 422 What Lidar data Looks Like Millions of XYZ points called a Lidar Point Cloud Area is approximately 1 X 075mi includes 440000 returns The raw lidar data is not an image Instead it is a cloud of millions of relative elevations Notice however that even with this lidar point cloud you can clearly make out patterns and features FOR 451 423 From Points to Inventory Delete all points except the last return which hopefully is from the ground or near to it There are a couple of ways people do this The easiest is with the Block Minimum method The Lidar points are divided into grid cells and the lowest point is chosen as the ground Image source Jeffery Evans RMRS In order to use the Lidar data we have to convert the relative elevations into absolute heights of trees and other objects We do this by rst de ning the elevation of the ground and then subtracting this value from all the values in the image Essentially we assume that trees rocks buildings etc will all have an elevation above the ground and thus the difference is their height Many different methods exist to determine the ground elevation from the Lidar point cloud The simplest include Slope threshold 7 all points above a de ned slope are deleted Block Minimum 7 the lowest point within a block is assume to be the ground Morphological Filters 7 Objects of certain shapes are deleted PCF progressive curvature lter 7 Objects with sharp edges are deleted Many others exist but whatever method is chosen it is typically an iterative process that repeats until the ground surface stops changing by a noticeable amount FOR 451 423 From Points to Inventory This process is repeated until the surface stops changing Image source Jeffery Evans RMRS This is an example of applying the Block Minimum method to a line ofLidar data As you can see the process continues until all the nonground Ie tree points have been removed leaving only the ground returns FOR 451 424 From Points to Terrain Before After quot A v mum smsn mesa swan 5min amen Image source Jeffery Evans RMRS This example shows both the raw lidar dataset and the derived Block Minimum surface FOR 451 Again we see the results of another ground elevation production method As you can see generating the ground elevation is an effective way of producing a digital elevation model Indeed alarge body of research uses Lidar for this purpose with the vegetation information commonly described by geologists as unnecessary noise It is however this noise that we are interested in As such if you are ever involved in a lidar acquisition make sure that all the data is given by the vendor to the land manager as many vendors will simply remove the data to make it a more manageable size FOR 451 425 Seeing the Trees To get the canopy height we simply subtract the surface produced from the ground returns from a surface produced from the non ground returns Canopy Returns minus Ground Returns Once the ground elevation is known we simply subtract this surface from each canopy return to produce a new surface called a canopy height model This is simply a surface of the absolute height of the forest canopy FOR 451 425 Seeing the Trees Imrage source HE Anderson USDA FS PNW Prior to any quantitative analysis we can assign colors to the lidar derived absolute heights green to high and brown to low to produce images representing the forest such as that shown here FOR 451 426 Sources of Error Minimal error Interpolation error ground ground 1 Scale error over Smoothing ground Dens1ty geometry and measurement Nongmund if Image source Jeffery Evans RMRS Before we can use Lidar to actually provide measures of fuels we rst need to consider some of the sources of potential error Errors can arise from interpolators missing points resulting in trees within surfaces being too large or too smal 39 There may be insuf cient returns so that the treetops middle or even the ground are not adequately sampled FOR 451 426 Sources of Error High scan angles can distort the LiDAR footprint Scan angle interacts with slope to add error by increasing distance Footprint Footprint Scan Angle Image source Jeffery Evans RMRS The trees may be on a slope or the Lidar pulse may be sent out at a wide angle producing errors in the relative elevations A FOR 451 427 Using Lidar for Fuels Inventory From structure we can get at Fire fuels Habitat timber etc Disadvantages do exist Solution Collect wall towall once and then for A J FOR 451 428 What You Should Know What are the advantages and disadvantages of field aerial photographs lidar and satellite measurements Why might the last lidar return not be from the ground How would you use lidar to produce a digital elevation model What are common sources of error associated with lidar
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