How do you get the highest accuracy from your system?
We’re often asked if ScanLook can compete with terrestrial laser scanning in regards to accuracy. Below we make an attempt to give an explanation of the technology and what can be achieved with it. Like so many other things, good people, good procedures, and a good day make a big difference in the end result.
The weakest component in any system is the one that doesn’t deliver. In a Mobile Mapping System (MMS) we have several key components but perhaps the two most important are the INS and the scanner(s). In this analysis we want to compare the expected relative and absolute accuracy of a MMS with a TLS (Terrestrial Laser Scanner).
ScanLook uses the FARO FOCUS 3D laser scanner for high definition, hard surface, high accuracy scanning. This scanner is rated as a 0.5 to 2.0 mm scanner. Through repeated tests it has been shown it is roughly a 1 mm scanner. Using the F3D in a static process on a tripod we can expect results similar to other scanners of equivalent quality. Basically 5 to 6 mm is a realistic expectation for a network scanned with the F3D. These results are all relative as we have not introduced any control points. If we introduce control points we would have to compute a full error propagation for the system. Let’s do something simple. Let’s assume the control is PERFECT or less than 1mm in absolute accuracy. Basically that leaves the TLS solution at 5 to 6 mm in relative and absolute accuracy. Of course, we know the control is not perfect but we’ll assume it is.
Moving along, the question is whether or not this same sort of result can be achieved from a mobile platform. Instead of being held on a solidly fixed tripod absolutely free from any disturbances along our survey our MMS is moving on a relatively smooth surface. There will be some abrupt changes during the travel in spite of this. The scanner is scanning in a 2D plane collecting points radially outward along the line of travel (BTW, this is a big advantage of the MMS such that the highest density of points is always nearest to the line of travel and consequent area of interest). Because we are really concerned with the dynamic nature of the scanner we want to compare scan lines in motion. This is where the IMU becomes so very important. Let's look more at what the IMU does.
The IMU measures two things: orientation (i.e. angles, attitude, or rotational position relative to the inertial coordinate system) and acceleration. The gyros measure the orientation and the accelerometers measure the inertial acceleration or G–forces. What is of key importance here is what can be expected to our "near perfect" laser recording along our path. The NovAtel ISA–100C can maintain 3mm accuracy at 25 meters. This is just slightly over what can be expected from the scanner. This establishes that the relative accuracy of the points in any particular scanline remain within just a few millimeters.
Now we need to address what happens along the line of travel. The heading accuracy degrades roughly by 2.5 times that of the roll and pitch. This equates to less than a centimeter over the same 25 meter distance. However, the heading and the GPS trajectory work in tandem. This is very important in the post–processing step. All of this, combined with the acceleration readings, generate the pathway of the trajectory. Basically this is good to around 1 to 2 cm assuming good GPS signal. Most important is that the IMU can monitor and account for small and very rapid changes by working at 200Hz. It is highly unlikely that a ground–based vehicle (outside of a helicopter sitting on the ground) could vibrate too fast for the IMU to accurately account for it.
At this point we are primary limited to our along track errors of 1 to 2 cm. Our experience has shown that we can map an open interstate with good GPS coverage at the +/– 1cm level in repeated test. Now we need to improve our results to 5 to 6 mm. This is where control points and good software become very important. The point cloud at this point is not dissimilar to old style photogrammetric data. In this case, we would employ control points with our point cloud data and use an appropriate math model to tie the observations to the ground (in photogrammetry we would have used a strip or block adjustment program such as PAT–M, SPACE–M, or ATP). By measuring corresponding points in the point cloud and adjusting them to the "perfect" control with a sufficient point density, we can adjust the point cloud to meet our specs. The key here is to have a sufficiently dense pattern of well distributed control points to support the accuracy needed.
How dense is dense enough? This depends upon the accuracy required. It may require control every 100, 250, or 500 feet. Please refer to this article for an independent study for control point dispersion and accuracy results.
In many cases involving static scanning each setup is dependent upon local control (i.e. control within range of that single scan). Many variables come in to play to answer this question and there is no single answer. GPS coverage and blockage play a very big part of this and it varies constantly. In any case, it remains far faster to set a few control points to control the mobile scan data than it does to collect what might be literally hundreds of features in the same area using conventional methods or even static scanning.
The bottom line is that ScanLook can match the accuracy of a terrestrial scanner, at least that of the FARO FOCUS 3D. To do this does require a sufficient number of control points, the proper registration software, good GPS, the top end IMU, good procedures, good people and a good day.
The weakest component in any system is the one that doesn’t deliver. In a Mobile Mapping System (MMS) we have several key components but perhaps the two most important are the INS and the scanner(s). In this analysis we want to compare the expected relative and absolute accuracy of a MMS with a TLS (Terrestrial Laser Scanner).
ScanLook uses the FARO FOCUS 3D laser scanner for high definition, hard surface, high accuracy scanning. This scanner is rated as a 0.5 to 2.0 mm scanner. Through repeated tests it has been shown it is roughly a 1 mm scanner. Using the F3D in a static process on a tripod we can expect results similar to other scanners of equivalent quality. Basically 5 to 6 mm is a realistic expectation for a network scanned with the F3D. These results are all relative as we have not introduced any control points. If we introduce control points we would have to compute a full error propagation for the system. Let’s do something simple. Let’s assume the control is PERFECT or less than 1mm in absolute accuracy. Basically that leaves the TLS solution at 5 to 6 mm in relative and absolute accuracy. Of course, we know the control is not perfect but we’ll assume it is.
Moving along, the question is whether or not this same sort of result can be achieved from a mobile platform. Instead of being held on a solidly fixed tripod absolutely free from any disturbances along our survey our MMS is moving on a relatively smooth surface. There will be some abrupt changes during the travel in spite of this. The scanner is scanning in a 2D plane collecting points radially outward along the line of travel (BTW, this is a big advantage of the MMS such that the highest density of points is always nearest to the line of travel and consequent area of interest). Because we are really concerned with the dynamic nature of the scanner we want to compare scan lines in motion. This is where the IMU becomes so very important. Let's look more at what the IMU does.
The IMU measures two things: orientation (i.e. angles, attitude, or rotational position relative to the inertial coordinate system) and acceleration. The gyros measure the orientation and the accelerometers measure the inertial acceleration or G–forces. What is of key importance here is what can be expected to our "near perfect" laser recording along our path. The NovAtel ISA–100C can maintain 3mm accuracy at 25 meters. This is just slightly over what can be expected from the scanner. This establishes that the relative accuracy of the points in any particular scanline remain within just a few millimeters.
Now we need to address what happens along the line of travel. The heading accuracy degrades roughly by 2.5 times that of the roll and pitch. This equates to less than a centimeter over the same 25 meter distance. However, the heading and the GPS trajectory work in tandem. This is very important in the post–processing step. All of this, combined with the acceleration readings, generate the pathway of the trajectory. Basically this is good to around 1 to 2 cm assuming good GPS signal. Most important is that the IMU can monitor and account for small and very rapid changes by working at 200Hz. It is highly unlikely that a ground–based vehicle (outside of a helicopter sitting on the ground) could vibrate too fast for the IMU to accurately account for it.
At this point we are primary limited to our along track errors of 1 to 2 cm. Our experience has shown that we can map an open interstate with good GPS coverage at the +/– 1cm level in repeated test. Now we need to improve our results to 5 to 6 mm. This is where control points and good software become very important. The point cloud at this point is not dissimilar to old style photogrammetric data. In this case, we would employ control points with our point cloud data and use an appropriate math model to tie the observations to the ground (in photogrammetry we would have used a strip or block adjustment program such as PAT–M, SPACE–M, or ATP). By measuring corresponding points in the point cloud and adjusting them to the "perfect" control with a sufficient point density, we can adjust the point cloud to meet our specs. The key here is to have a sufficiently dense pattern of well distributed control points to support the accuracy needed.
How dense is dense enough? This depends upon the accuracy required. It may require control every 100, 250, or 500 feet. Please refer to this article for an independent study for control point dispersion and accuracy results.
In many cases involving static scanning each setup is dependent upon local control (i.e. control within range of that single scan). Many variables come in to play to answer this question and there is no single answer. GPS coverage and blockage play a very big part of this and it varies constantly. In any case, it remains far faster to set a few control points to control the mobile scan data than it does to collect what might be literally hundreds of features in the same area using conventional methods or even static scanning.
The bottom line is that ScanLook can match the accuracy of a terrestrial scanner, at least that of the FARO FOCUS 3D. To do this does require a sufficient number of control points, the proper registration software, good GPS, the top end IMU, good procedures, good people and a good day.
What about typical accuracies?
What are typical mobile mapping accuracies?
We are often asked what accuracy we can deliver with ScanLook. There is no simple answer. The accuracy depends upon the specific hardware (INS & scanner), the GPS constellation, GPS outage, control points, post processing software and the personnel.
Accuracy is generally divided into relative and absolute accuracy. Even more, we can consider the accuracy of the roadway (or near the vehicle) and also of the more distant feature data (off the road such as buildings, poles, etc.).
Generally when we are asked about accuracy it is in reference to the roadway for survey grade data. In this case if we assume all of the data is very close to the scanner (20 or 30 feet) and that the scanner is perfect, then our results are limited by GPS positioning. Most often this is 2 to 4 cm (just over 0.1 ft) vertically and half of that horizontally. This is the best one could reliably offer to provide as an absolute accuracy using only INS data (GPS, GLONASS, etc. along with an IMU) for positioning.
If results better than this are required then control points are essential. The control points should be placed as near the line of travel as possible. On a busy roadway this usually means on the inside and outside shoulder to mitigate the danger to the surveyor. If possible a point per lane would be nearly ideal. If GPS accuracy is sufficient for XY positioning, or if highway stationing is good enough, then Z only points can be used. In the former case targets of sufficient size are necessary and in the latter case no targets are necessary. Control can be placed using total stations and/or levels for the best vertical accuracy. In any case, control should be placed on surfaces that are planar with at least a 1 ft radius. This could be a roadway (but not a curb), a manhole (in the center), or a wall (but not near a window or sharp corner).
The distance between control points required is not so much a function of distance itself but the speed of the vehicle during collection. If you are limited to 10mph then you will need more points than when traveling at 40mph. Keep in mind that your necessary target size is also a function of the speed you travel. If you are using a single laser scanner rotating at 100 Hz then at 50mph this equates to a separation of roughly 9 inches between scan lines. So your XY resolution will be determined by your speed, target size, shape, orientation and software.
Again, if you have good GPS (i.e. little or no outages and a good constellation) then you can expect accuracies from your mobile mapping system to match that of the GPS at short ranges. This assumes you have a reasonably good INS. To maintain that accuracy at a greater distance will require a good INS (one with a good roll and pitch resolution), a good scanner (one that is accurate and reliable without a major degradation as the range increases), and a well calibrated system.
There is no definitive answer. With a good system and using control, 0.05 feet results are possible, on or near the roadway, but not generally at an extended distance from the vehicle.
We are often asked what accuracy we can deliver with ScanLook. There is no simple answer. The accuracy depends upon the specific hardware (INS & scanner), the GPS constellation, GPS outage, control points, post processing software and the personnel.
Accuracy is generally divided into relative and absolute accuracy. Even more, we can consider the accuracy of the roadway (or near the vehicle) and also of the more distant feature data (off the road such as buildings, poles, etc.).
Generally when we are asked about accuracy it is in reference to the roadway for survey grade data. In this case if we assume all of the data is very close to the scanner (20 or 30 feet) and that the scanner is perfect, then our results are limited by GPS positioning. Most often this is 2 to 4 cm (just over 0.1 ft) vertically and half of that horizontally. This is the best one could reliably offer to provide as an absolute accuracy using only INS data (GPS, GLONASS, etc. along with an IMU) for positioning.
If results better than this are required then control points are essential. The control points should be placed as near the line of travel as possible. On a busy roadway this usually means on the inside and outside shoulder to mitigate the danger to the surveyor. If possible a point per lane would be nearly ideal. If GPS accuracy is sufficient for XY positioning, or if highway stationing is good enough, then Z only points can be used. In the former case targets of sufficient size are necessary and in the latter case no targets are necessary. Control can be placed using total stations and/or levels for the best vertical accuracy. In any case, control should be placed on surfaces that are planar with at least a 1 ft radius. This could be a roadway (but not a curb), a manhole (in the center), or a wall (but not near a window or sharp corner).
The distance between control points required is not so much a function of distance itself but the speed of the vehicle during collection. If you are limited to 10mph then you will need more points than when traveling at 40mph. Keep in mind that your necessary target size is also a function of the speed you travel. If you are using a single laser scanner rotating at 100 Hz then at 50mph this equates to a separation of roughly 9 inches between scan lines. So your XY resolution will be determined by your speed, target size, shape, orientation and software.
Again, if you have good GPS (i.e. little or no outages and a good constellation) then you can expect accuracies from your mobile mapping system to match that of the GPS at short ranges. This assumes you have a reasonably good INS. To maintain that accuracy at a greater distance will require a good INS (one with a good roll and pitch resolution), a good scanner (one that is accurate and reliable without a major degradation as the range increases), and a well calibrated system.
There is no definitive answer. With a good system and using control, 0.05 feet results are possible, on or near the roadway, but not generally at an extended distance from the vehicle.
