| 1 | | |
| 2 | | |
| 3 | | == Ground classification == |
| 4 | | |
| 5 | | Classification is the process of labelling the points in the point cloud by type. Ground classification specifically refers to labelling points that're considered to be part of the ground(!) This is a required step for the automatic boresight procedure. |
| 6 | | |
| 7 | | Steps: |
| 8 | | * remove very low, noisy points from consideration first |
| 9 | | * e.g. remove groups of up to 3 points > 1m below neighbours (iterate a few times so it settles) |
| 10 | | * classify groups of low points as ground (run N times to settle), takes account of neighbours |
| 11 | | * repeat with singletons |
| 12 | | * remove isolated points (>5m from any other point) |
| 13 | | * manually "add points to ground" for areas that should be ground but haven't been classified as such (e.g. peaks of hills) and rerun process above |
| 14 | | |
| 15 | | Run as a macro. Terra->tools->macro->new. |
| 16 | | |
| 17 | | |
| 18 | | == QC procedures == |
| | 1 | = LIDAR processing notes = |
| | 2 | |
| | 3 | Notes from visit to Leica, Switzerland on 11/Aug/2008. |
| | 4 | |
| | 5 | Contact email: |
| | 6 | * sensors_emea@leica-geosystems.com |
| | 7 | * Various support engineers at Leica. The one that trained us was Barbara Haebler. |
| | 8 | |
| | 9 | FTP site (for updates, etc): |
| | 10 | * ftp://airborne:LsA36GmW@ftp.leica-geosystems/airborne/ |
| | 11 | * problem uploads to go to incoming/ dir |
| | 12 | |
| | 13 | == Things that may go wrong == |
| | 14 | |
| | 15 | === In flight === |
| | 16 | |
| | 17 | Flying too low - if the LIDAR detects that the laser power may be too high for eye safety, it will cut out the laser automatically (if you're a seagull looking up, bad luck, it only accounts for ground height). |
| | 18 | |
| | 19 | Flying outside the "range gate" (acceptable ranges of distances) may cause similar effects. Going too high will tend to make the edges of the swath drop out first (due to path length). |
| | 20 | |
| | 21 | Automatic gain control weirdness - the measured intensity is returned via an AGC which may step up or down depending on the returns from the ground. Measured return intensity should only be used as a guideline rather than a real measurement. |
| | 22 | |
| | 23 | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| | 24 | |
| | 25 | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| | 26 | |
| | 27 | === In processing === |
| | 28 | |
| | 29 | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| | 30 | |
| | 31 | Streaky intensity images - "bad, contact support for guidance". These will probably cause range errors due to the intensity variation, no idea what the cause would be as yet.. |
| | 32 | |
| | 33 | Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges. |
| | 34 | |
| | 35 | === Other error sources === |
| | 36 | |
| | 37 | Atmospheric effects; the light path may bend due to atmospheric refraction, density effects, etc causing the laser to hit the ground earlier or later than expected, or in a different position. This will be most noticeable at the edges of the swath where there are longer path lengths (and more atmosphere to pass through) and may look like the swath curls up or down at the edges (path length error), or may look like compression or stretching of the edges (if there is horizontal divergence). The error was claimed to be a fraction of a meter at 6km altitude (i.e. not well bounded). Measurement of temperature may help with this effect, but was said to be a minor value. |
| | 38 | |
| | 39 | Range correction error; if the range correction is wrong, the electronics will measure the path length incorrectly. Points at nadir will be in error vertically only and points at the edge will have a vertical and horizontal error. The error will make a flat piece of land look like a smile and the nadir point will be below the expected land surface (one can only get "late" measurements?). |
| | 40 | |
| | 41 | Torsion (of mirror) error; the mirror may be out of the expected position at the edges of the swath due to it bending under high acceleration. There will be no error at nadir (no acceleration) and errors at the edges, inducing a smile effect again, but with the nadir at the correct height. Range errors should be corrected before working on the torsion error, and the nadir point should be used for the range correction as there will be no mirror torsion effect there. |
| | 42 | |
| | 43 | MPiA mode errors: if a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up. On an edge of a very unluckily placed cloud, this would look a bit like the cloud merging into the ground. Presumably rare or minor. |
| | 44 | |
| | 45 | |
| | 46 | == Mission planning considerations == |
| | 47 | |
| | 48 | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| | 49 | |
| | 50 | For differential GPS, aim to have a base station nearby (within 20km) and ideally in center of the scene. Base station data should be recorded at 2Hz |
| | 51 | |
| | 52 | Ideally choose a time with a good GPS constellation (lots of satellites, PDOP >= 4). |
| | 53 | |
| | 54 | Consider terrain and minimum target size to determine required point density. Steep terrain may cause shadowing effects due to perspective. |
| | 55 | |
| | 56 | Consider reflectivity of surface - lighter surfaces need less laser power. Too high a power means intensity overflows on the reflection, which mess up the range - AGC should deal with this for large areas but it should be considered in planning, particularly for small bright targets on a generally darker background. Assume 10% reflectivity is typical. |
| | 57 | |
| | 58 | Recommended to do a figure 8 loop at the start and end of the data acquisitions to make the IMU happy. Need one at the end for reverse navigation processing. |
| | 59 | |
| | 60 | == Overall system design and comments of note == |
| | 61 | |
| | 62 | System is a Leica ALS50 (phase II) LIDAR. There is an accompanying 39 megapixel [7216x5412, 12bit] digital camera, referred to as the "RCD". |
| | 63 | |
| | 64 | A GPS receiver and IMU (brand?) are included. The LIDAR, RCD and IMU, are mounted on a shock plate to protect it from strong movements - this will also isolate the IMU from the hyperspectral imaging sensors. |
| | 65 | |
| | 66 | The IPAS controller (name correct?) allows for event recording, amongst other things. Phil plans to use this to record frame sync pulses from Eagle and Hawk. |
| | 67 | |
| | 68 | There are also some associated control and display devices for operator and pilot usage. |
| | 69 | |
| | 70 | === How it works === |
| | 71 | |
| | 72 | The LIDAR works by firing a (4ns or 9ns) laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance. The pulse isn't modulated by a carrier - it's just an on/off pulse. There are four timing cards ("range cards R1-R4") running for a pulse, so up to 4 returns can be detected (R4 actually detects the last return rather than the 4th?). The system has a "MPiA" (Multiple Pulses in the Air) mode, which fires two pulses evenly separated, rather than waiting for the first to come back before firing another [SPiA mode, times out in case the pulse is eaten]. To measure this, there are actually two banks of timing cards (bank A and bank B, both with R1-R4 cards), so there are 8 timing cards in total. |
| | 73 | |
| | 74 | A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent. The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain. When there are 4 returns, each range card measures the time of the return pulse. When there are less than 4 returns, R4 is a second measurement (not a copy of) of the last pulse - i.e. if there are 2 returns, you will have R1, R2 and R4 (= re-measurement of R2). |
| | 75 | |
| | 76 | The intensity of a return is measured only for the first 3 returns (R1-R3), and is an 8 bit value (0=dark (water), 255=bright) relating to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller, and is not related to a physical measure (can it be?). The intensity can be used in various processing algorithms to help distinguish transitions between surfaces. The AGC tries to keep the intensity in the range 100-150 or so. |
| | 77 | |
| | 78 | The laser is scanned across a (up-to) 75 degree swath by an oscillating mirror. Due to the acceleration/deceleration of the mirror, this produces a sinusoidal pattern to the trace on the ground, with the highest density of points at the peak and trough of the sine wave (i.e. at the edges of the swath). If the swath width is set to less than 75 degrees (45 degrees recommended), there's a roll compensation mechanism that tries to smooth out small roll movements by using the remaining freedom of motion. The scan rate of the mirror is dependent on the FOV angle (36Hz for 40 degree FOV, 24Hz for a 75 degree FOV, calculate with 412.33 * FOV^-0.6548). |
| | 79 | |
| | 80 | The laser is an 8W class 4 laser, operating in the infrared range. The divergence results in approximately a 22cm spot on the ground when fired from 1000m up. |
| | 81 | |
| | 82 | Controllable parameters: |
| | 83 | * laser intensity (0 -> 8W output), controlled by operator as a percentage output. Has safety cutouts if the light level at the ground could cause eye damage. |
| | 84 | |
| | 85 | * altitude (kinda a parameter ;) ) - minmium of ~650 up to ~2000m (after 2km, you start getting poor returns on forests, etc, the real limit is up to about 6km in ideal conditions) |
| | 86 | * ... pulse frequency, scan angle, etc [TBD] |
| | 87 | |
| | 88 | === Automatic Gain Control === |
| | 89 | |
| | 90 | The intensity of a return is measured (R1-R3) as an 8 bit value (0=dark (water), 255=bright) relating to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller to keep it within 8 bit range. The value of the AGC is also measured as an 8 bit value and is recorded per pulse as the "gain control voltage" in the raw files. Typical values are 110-180. |
| | 91 | |
| | 92 | The AGC is controlled automatically in our LIDAR, but operates on a threshold basis. If there are N (64?) low intensity points in a row, the AGC will step up one level. If there are M (32?) overbright points in a row, the AGC will step down. This may lead to visible steps in the image intensity, but isn't supposed to be a big problem. Look out for streaky areas though ("contact support if you see this" - though not sure why). |
| | 93 | |
| | 94 | |
| | 95 | == RCD camera == |
| | 96 | |
| | 97 | The system has a 39 megapixel digital camera, referred to as the "RCD", with the following characteristics: |
| | 98 | * 7216x5412 resolution, 12bit [=~57MB raw] |
| | 99 | * 60mm lenses (changeable), with 44.2 degree x 34 degree FOV |
| | 100 | * pixel size 6.8microns (= ~15cm ground resolution at 1350m) |
| | 101 | * 1/4000 exposure time, 2.2s per frame |
| | 102 | * serial number 21 |
| | 103 | * shutter is curtain style, taking 8ms to open and close, so there's a 4ms time difference between the top and bottom of the CCD stopping acquiring light |
| | 104 | |
| | 105 | Logfiles contain various handy info, notably the image event file contains GPS time, pos/att and picture id |
| | 106 | |
| | 107 | Calibration files are required and supplied by Leica (dark, gain and camera parameters). |
| | 108 | |
| | 109 | File name convention is: `DDHHMMSSXXXXXXGN.raw` |
| | 110 | * DD = day of month |
| | 111 | * HHMMSS = timestamp |
| | 112 | * XXXXXX = incrementing picture id |
| | 113 | * G = gain (1-4) |
| | 114 | |
| | 115 | === Orthophotos === |
| | 116 | |
| | 117 | This apparently takes another week long training course. We do have the basic software to do it but limited knowledge. Basic process is to determine the camera parameters, correct and overlay the images, keeping ideally only the most central parts. You then have to manually check through, especially along seams, and manually move the seam line in some instances (e.g. when perspective effects result in odd errors due to the seam hitting a tall building). |
| | 118 | |
| | 119 | |
| | 120 | == Recommendations == |
| | 121 | |
| | 122 | Perform calibrations frequently at the start of the period to get a feel for how they hold. Reduce down to fit circumstances over time. |
| | 123 | |
| | 124 | == Required items for processing == |
| | 125 | |
| | 126 | Items required from every flight: |
| | 127 | * GPS and IMU data (including a basestation if not using PPP) |
| | 128 | * Raw laser data |
| | 129 | * Logfiles from LIDAR |
| | 130 | * Flight logfiles are useful too, if the LIDAR flight planning software is used |
| | 131 | * RCD & webcam images |
| | 132 | * Pressure and temperature measurements at the plane position above the site [this affects how long it takes the laser light to move through the air] |
| | 133 | |
| | 134 | Items required in general: |
| | 135 | * calibration (see elsewhere) |
| | 136 | * lever arm measurements |
| | 137 | |
| | 138 | = Processing = |
| | 139 | == Extraction == |
| | 140 | |
| | 141 | Use IPAS Pro to extract GPS, IMU and laser data from the raw files. |
| | 142 | * probably worth having the real time navigation info (doesn't add a lot of processing time) |
| | 143 | * during extraction, view the listing and verify the lever arms are correct (IMU one should never change). |
| | 144 | * look out for data gaps (listed in log) |
| | 145 | |
| | 146 | |
| | 147 | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| | 148 | |
| | 149 | == Navigation processing == |
| | 150 | |
| | 151 | See other pages on navigation processing as they cover this already. |
| | 152 | |
| | 153 | |
| | 154 | == Initial QC == |
| | 155 | |
| | 156 | See if the lines are too short or if the point cloud has poor return %s. If so, check the following (look at webcam images for hints): |
| | 157 | * clouds |
| | 158 | * height problem ("range gate" issue) |
| | 159 | * eyesafe shutoff (too close to ground) |
| | 160 | * too high may give dropouts |
| | 161 | * find altitude over ground (measured GPS alt - geoid-spheroid height) and see if it's within the min & max ranges |
| | 162 | |
| | 163 | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| | 164 | * filter out incorrect points (based on high intensity and height difference from locality?) |
| | 165 | * also, if you observe odd spikes, check for intensity over 150 - this may indicate a two-peaked response with the first peak being 150 (and being the detected one) and the second being 255 (overflow), causing the effect above |
| | 166 | |
| | 167 | |
| | 168 | == Full QC procedures == |
| 31 | | == Automatic Gain Control == |
| 32 | | |
| 33 | | The intensity of a return is measured (R1-R3) as an 8 bit value (0=dark (water), 255=bright) relating to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller to keep it within 8 bit range. The value of the AGC is also measured as an 8 bit value and is recorded per pulse as the "gain control voltage" in the raw files. Typical values are 110-180. |
| 34 | | |
| 35 | | The AGC is controlled automatically in our LIDAR, but operates on a threshold basis. If there are N (64?) low intensity points in a row, the AGC will step up one level. If there are M (32?) overbright points in a row, the AGC will step down. This may lead to visible steps in the image intensity, but isn't supposed to be a big problem. Look out for streaky areas though ("contact support if you see this" - though not sure why). |
| 36 | | |
| 37 | | == RCD camera == |
| 38 | | |
| 39 | | The system has a 39 megapixel digital camera, referred to as the "RCD", with the following characteristics: |
| 40 | | * 7216x5412 resolution, 12bit [=~57MB raw] |
| 41 | | * 60mm lenses (changeable), with 44.2 degree x 34 degree FOV |
| 42 | | * pixel size 6.8microns (= ~15cm ground resolution at 1350m) |
| 43 | | * 1/4000 exposure time, 2.2s per frame |
| 44 | | * serial number 21 |
| 45 | | * shutter is curtain style, taking 8ms to open and close, so there's a 4ms time difference between the top and bottom of the CCD stopping acquiring light |
| 46 | | |
| 47 | | Logfiles contain various handy info, notably the image event file contains GPS time, pos/att and picture id |
| 48 | | |
| 49 | | Calibration files are required and supplied by Leica (dark, gain and camera parameters). |
| 50 | | |
| 51 | | File name convention is: `DDHHMMSSXXXXXXGN.raw` |
| 52 | | * DD = day of month |
| 53 | | * HHMMSS = timestamp |
| 54 | | * XXXXXX = incrementing picture id |
| 55 | | * G = gain (1-4) |
| 56 | | |
| 57 | | === Orthophotos === |
| 58 | | |
| 59 | | This apparently takes another week long training course. We do have the basic software to do it but limited knowledge. Basic process is to determine the camera parameters, correct and overlay the images, keeping ideally only the most central parts. You then have to manually check through, especially along seams, and manually move the seam line in some instances (e.g. when perspective effects result in odd errors due to the seam hitting a tall building). |
| 60 | | |
| 61 | | = LIDAR processing notes = |
| 62 | | |
| 63 | | Notes from visit to Leica, Switzerland on 11/Aug/2008. |
| 64 | | |
| 65 | | Contact email: |
| 66 | | * sensors_emea@leica-geosystems.com |
| 67 | | * Various support engineers at Leica. The one that trained us was Barbara Haebler. |
| 68 | | |
| 69 | | FTP site (for updates, etc): |
| 70 | | * ftp://airborne:LsA36GmW@ftp.leica-geosystems/airborne/ |
| 71 | | * problem uploads to go to incoming/ dir |
| 72 | | |
| 73 | | == Things that may go wrong == |
| 74 | | |
| 75 | | === In flight === |
| 76 | | |
| 77 | | Flying too low - if the LIDAR detects that the laser power may be too high for eye safety, it will cut out the laser automatically (if you're a seagull looking up, bad luck, it only accounts for ground height). |
| 78 | | |
| 79 | | Flying outside the "range gate" (acceptable ranges of distances) may cause similar effects. Going too high will tend to make the edges of the swath drop out first (due to path length). |
| 80 | | |
| 81 | | Automatic gain control weirdness - the measured intensity is returned via an AGC which may step up or down depending on the returns from the ground. Measured return intensity should only be used as a guideline rather than a real measurement. |
| 82 | | |
| 83 | | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| 84 | | |
| 85 | | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| 86 | | |
| 87 | | === In processing === |
| 88 | | |
| 89 | | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| 90 | | |
| 91 | | Streaky intensity images - "bad, contact support for guidance". These will probably cause range errors due to the intensity variation, no idea what the cause would be as yet.. |
| 92 | | |
| 93 | | Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges. |
| 94 | | |
| 95 | | === Other error sources === |
| 96 | | |
| 97 | | Atmospheric effects; the light path may bend due to atmospheric refraction, density effects, etc causing the laser to hit the ground earlier or later than expected, or in a different position. This will be most noticeable at the edges of the swath where there are longer path lengths (and more atmosphere to pass through) and may look like the swath curls up or down at the edges (path length error), or may look like compression or stretching of the edges (if there is horizontal divergence). The error was claimed to be a fraction of a meter at 6km altitude (i.e. not well bounded). Measurement of temperature may help with this effect, but was said to be a minor value. |
| 98 | | |
| 99 | | Range correction error; if the range correction is wrong, the electronics will measure the path length incorrectly. Points at nadir will be in error vertically only and points at the edge will have a vertical and horizontal error. The error will make a flat piece of land look like a smile and the nadir point will be below the expected land surface (one can only get "late" measurements?). |
| 100 | | |
| 101 | | Torsion (of mirror) error; the mirror may be out of the expected position at the edges of the swath due to it bending under high acceleration. There will be no error at nadir (no acceleration) and errors at the edges, inducing a smile effect again, but with the nadir at the correct height. Range errors should be corrected before working on the torsion error, and the nadir point should be used for the range correction as there will be no mirror torsion effect there. |
| 102 | | |
| 103 | | MPiA mode errors: if a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up. On an edge of a very unluckily placed cloud, this would look a bit like the cloud merging into the ground. Presumably rare or minor. |
| 104 | | |
| 105 | | |
| 106 | | == Mission planning considerations == |
| 107 | | |
| 108 | | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| 109 | | |
| 110 | | For differential GPS, aim to have a base station nearby (within 20km) and ideally in center of the scene. Base station data should be recorded at 2Hz |
| 111 | | |
| 112 | | Ideally choose a time with a good GPS constellation (lots of satellites, PDOP >= 4). |
| 113 | | |
| 114 | | Consider terrain and minimum target size to determine required point density. Steep terrain may cause shadowing effects due to perspective. |
| 115 | | |
| 116 | | Consider reflectivity of surface - lighter surfaces need less laser power. Too high a power means intensity overflows on the reflection, which mess up the range - AGC should deal with this for large areas but it should be considered in planning, particularly for small bright targets on a generally darker background. Assume 10% reflectivity is typical. |
| 117 | | |
| 118 | | Recommended to do a figure 8 loop at the start and end of the data acquisitions to make the IMU happy. Need one at the end for reverse navigation processing. |
| 119 | | |
| 120 | | == Overall system design and comments of note == |
| 121 | | |
| 122 | | System is a Leica ALS50 (phase II) LIDAR. There is an accompanying 39 megapixel [7216x5412, 12bit] digital camera, referred to as the "RCD". |
| 123 | | |
| 124 | | A GPS receiver and IMU (brand?) are included. The LIDAR, RCD and IMU, are mounted on a shock plate to protect it from strong movements - this will also isolate the IMU from the hyperspectral imaging sensors. |
| 125 | | |
| 126 | | The IPAS controller (name correct?) allows for event recording, amongst other things. Phil plans to use this to record frame sync pulses from Eagle and Hawk. |
| 127 | | |
| 128 | | There are also some associated control and display devices for operator and pilot usage. |
| 129 | | |
| 130 | | === How it works === |
| 131 | | |
| 132 | | The LIDAR works by firing a (4ns or 9ns) laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance. The pulse isn't modulated by a carrier - it's just an on/off pulse. There are four timing cards ("range cards R1-R4") running for a pulse, so up to 4 returns can be detected (R4 actually detects the last return rather than the 4th?). The system has a "MPiA" (Multiple Pulses in the Air) mode, which fires two pulses evenly separated, rather than waiting for the first to come back before firing another [SPiA mode, times out in case the pulse is eaten]. To measure this, there are actually two banks of timing cards (bank A and bank B, both with R1-R4 cards), so there are 8 timing cards in total. |
| 133 | | |
| 134 | | A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent. The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain. When there are 4 returns, each range card measures the time of the return pulse. When there are less than 4 returns, R4 is a second measurement (not a copy of) of the last pulse - i.e. if there are 2 returns, you will have R1, R2 and R4 (= re-measurement of R2). |
| 135 | | |
| 136 | | The intensity of a return is measured only for the first 3 returns (R1-R3), and is an 8 bit value (0=dark (water), 255=bright) relating to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller, and is not related to a physical measure (can it be?). The intensity can be used in various processing algorithms to help distinguish transitions between surfaces. The AGC tries to keep the intensity in the range 100-150 or so. |
| 137 | | |
| 138 | | The laser is scanned across a (up-to) 75 degree swath by an oscillating mirror. Due to the acceleration/deceleration of the mirror, this produces a sinusoidal pattern to the trace on the ground, with the highest density of points at the peak and trough of the sine wave (i.e. at the edges of the swath). If the swath width is set to less than 75 degrees (45 degrees recommended), there's a roll compensation mechanism that tries to smooth out small roll movements by using the remaining freedom of motion. The scan rate of the mirror is dependent on the FOV angle (36Hz for 40 degree FOV, 24Hz for a 75 degree FOV, calculate with 412.33 * FOV^-0.6548). |
| 139 | | |
| 140 | | The laser is an 8W class 4 laser, operating in the infrared range. The divergence results in approximately a 22cm spot on the ground when fired from 1000m up. |
| 141 | | |
| 142 | | Controllable parameters: |
| 143 | | * laser intensity (0 -> 8W output), controlled by operator as a percentage output. Has safety cutouts if the light level at the ground could cause eye damage. |
| 144 | | |
| 145 | | * altitude (kinda a parameter ;) ) - minmium of ~650 up to ~2000m (after 2km, you start getting poor returns on forests, etc, the real limit is up to about 6km in ideal conditions) |
| 146 | | * ... pulse frequency, scan angle, etc [TBD] |
| 147 | | |
| 148 | | == Recommendations == |
| 149 | | |
| 150 | | Perform calibrations frequently at the start of the period to get a feel for how they hold. Reduce down to fit circumstances over time. |
| 151 | | |
| 152 | | == Required items for processing == |
| 153 | | |
| 154 | | Items required from every flight: |
| 155 | | * GPS and IMU data (including a basestation if not using PPP) |
| 156 | | * Raw laser data |
| 157 | | * Logfiles from LIDAR |
| 158 | | * Flight logfiles are useful too, if the LIDAR flight planning software is used |
| 159 | | * RCD & webcam images |
| 160 | | * Pressure and temperature measurements at the plane position above the site [this affects how long it takes the laser light to move through the air] |
| 161 | | |
| 162 | | Items required in general: |
| 163 | | * calibration (see elsewhere) |
| 164 | | * lever arm measurements |
| 165 | | |
| 166 | | = Processing = |
| 167 | | == Extraction == |
| 168 | | |
| 169 | | Use IPAS Pro to extract GPS, IMU and laser data from the raw files. |
| 170 | | * probably worth having the real time navigation info (doesn't add a lot of processing time) |
| 171 | | * during extraction, view the listing and verify the lever arms are correct (IMU one should never change). |
| 172 | | * look out for data gaps (listed in log) |
| 173 | | |
| 174 | | |
| 175 | | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| 176 | | |
| 177 | | == Navigation processing == |
| 178 | | |
| 179 | | See other pages on navigation processing as they cover this already. |
| 180 | | |
| 181 | | |
| 182 | | == Initial QC == |
| 183 | | |
| 184 | | See if the lines are too short or if the point cloud has poor return %s. If so, check the following (look at webcam images for hints): |
| 185 | | * clouds |
| 186 | | * height problem ("range gate" issue) |
| 187 | | * eyesafe shutoff (too close to ground) |
| 188 | | * too high may give dropouts |
| 189 | | * find altitude over ground (measured GPS alt - geoid-spheroid height) and see if it's within the min & max ranges |
| 190 | | |
| 191 | | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| 192 | | * filter out incorrect points (based on high intensity and height difference from locality?) |
| 193 | | * also, if you observe odd spikes, check for intensity over 150 - this may indicate a two-peaked response with the first peak being 150 (and being the detected one) and the second being 255 (overflow), causing the effect above |