Airborne 1 Corporation
Briefing Note
Airborne 1 Corporation
BN#01
ABOUT AIRBORNE 1 CORPORATION 300 N. Sepulveda Blvd. Suite 1060 El Segundo, CA 90245 USA Phone: 310.414.7400 Fax: 310.414.7409 E-mail: info@airborne1.com
We’re on the web! www.airborne1.com
Airborne 1 Corporation provides flexible access to advanced LiDAR technology for partners in the photogrammetry, surveying and mapping fields. Airborne 1's digital mapping services and solutions include a dedicated airborne LiDAR survey group operating Optech ALTM sensors; flexible fractional ownership plans for partners without dedicated access to LiDAR technology; LiDAR data processing, analysis and application development; LiDAR field survey coordination and project management. Airborne 1 was incorporated in 1998, the same year its management team took home the coveted USC MBA "Best Business Plan" award.
Flexible. Accessible. Affordable.
link [‘li(ng)k] n.: a connecting structure, element or factor
LIDAR ACCURACY AIRBORNE 1’ S BIMONTHLY LIDAR PUBLICATION THE LIDAR NETWORK
DECEMBER 2002 / VOL.. 1 NO. 4
AN AIRBORNE 1 PERSPECTIVE
The Link [‘(th)& ‘li(ng)k] nl.: your connection to the LiDAR industry Send your e-mail to subscribe@airborne1.com and ask for your FREE SUBSCRIPTION to our newsletter, THE LINK!
LiDARing the World Everything you need to know before surveying abroad
Baseline Length in LiDAR Surveys Achieving Profitability and Keeping Accuracy
LiDAR & Forestry Changes to the forestry market are right around the corner
CONTENTS
Get linked to T H E
L I D A R
N E T W O R K
This publication is the second in a series of Briefing Notes we will be releasing in the coming months. If you’d like to subscribe to this series, send an e-mail to briefing@airborne1.com with the subject line “subscribe”.
Airborne 1 Corporation 300 N. SEPULVEDA BLVD. #1060, EL SEGUNDO, CA 90245 PHN. 310.414.7400 / FAX 310.414.7409
LAidi ar r bAcc o r ur n eac1 y C o r po r at i o n
Page 1 Briefing Note BN#01
BA CKG ROUND
I NS ID E THI S B RI EF IN G:
Flexible …
Background Technical Specifications Project #1 Laser Error Project #2 GPS Error IMU Error Project #3 Project #4 Standards Project #5 Summary References
Accessible …
Affordable
3 3 5 6 8 8 11 12 12 15 16 19 21 21
Special points of interest: •
An overview of the issues affecting lidar accuracy.
•
Discussion of range, position, and orientation errors.
•
Results from our last 5 projects
•
Lidar and mapping standards.
There is growing concern and confusion in the lidar end user community about the achievable accuracy of airborne lidar data and how this accuracy impacts products certified to various established mapping Lidar DSM of San Francisco by Airborne 1. standards. The purpose of this briefing note conducted using an is to provide background Optech ALTM 1225. It is information and discussion important when discussing points for lidar users in order lidar accuracy to keep in to better educate them about mind that the theoretical lidar accuracy specifications. system error based on a A general discussion of rigorous engineering system design error budgets analysis of the system is is complemented with generally not achievable in examples from recent the field. commercial surveys we have (Continued on page 5)
TEC HNIC AL CONSID ER ATI ONS: E R RO R B UDG ET F OR LID A R The error budget for a given airborne lidar mapping system is primarily driven by the contributing error budgets from the core subsystems; the laser rangefinder, the GPS position
solution and the IMU orientation solution. Contributions to the error budget include such diverse factors as the inherent pointing error of the laser, inaccuracies (Continued on page 4)
Page Page 14
B r ie f ing N o te
Page Page 15
T EC HNI C AL C ON SI DE R ATI ON S : ERROR BU DGE T F OR LI DA R (Continued from page 3)
“extremely accurate IMUs would not necessarily improve pointing accuracy if scanner angle measurements are still made to only 0.5 accuracy”
in the response time (latency) of the receiver electronics, view angle mismatch between the transmitter and receiver, sensor mounting biases - which are the small angular misalignments between the laser reference frame and the IMU reference frame inaccuracies in measuring the lever arm (antenna offset) between the GPS antenna phase center and the reference point of the laser output usually taken to be the center of the output mirror - and the error inherent in recording the scanner angle at the moment of each pulse. In considering error budgets for a given lidar sensor it is important to understand that the final accuracy will be limited by the convolution of all contributors to the error budget. System engineers need to balance each subsystems contribution against desired system performance to avoid price/performance discrepancies. For example, a laser
BA CK GRO U ND Operational considerations, such as variations in GPS quality, will significantly affect the final accuracy. In addition an end user or contracting agency will be faced with various interpretations – and misinterpretations – of what is meant by the accuracy of the lidar data. Service providers are often not clear about how stated accuracies will vary under different conditions across the project, for example in areas of steep slope, or how the stated accuracy will be quality checked and
rangefinder with subcentimeter accuracy would not necessarily improve overall system performance if GPS positioning accuracy were not improved; an extremely accurate IMU will not necessarily improve pointing accuracy if scanner angle measurements are still made to only 0.5° accuracy. For an excellent discussion and detailed examples of how each system parameter contributes to overall lidar system accuracy, see Baltsavias (1999a). For an independently published analysis of lidar accuracy using commercial sensors, see Kraus & Pfeifer (1998). Shrestha et. al. (2000) and Gutierrez et. al. (1998).
B r ie f ing N o te
controlled. Lidar system manufacturers are often vague about the conditions under which system specifications apply and are generally known to quote specifications that are best case or averaged across expected results. At Airborne 1 we feel an educated end user will be able to make better decisions about lidar as a suitable tool for their project. We hope these briefing notes will help.
S Y S TEM AC CURA CY S PE CI FI CA TI ON S
“The accuracy specifications quoted by LiDAR system manufacturers and service providers are currently a source of significant debate within the community.”
While the error budget for a particular lidar system can be reasonably well defined by proper system engineering analysis that considers the inherent inaccuracies of its components, the final operational accuracy that can be achieved is generally worse than the theoretical limit and hence more difficult to specify and open to interpretation. The accuracy specifications quoted by lidar system manufacturers and service providers are currently a source of significant debate within the community, especially amongst end users of lidar data. This is in part due to a lack of clear definitions of what is meant when
stating accuracy for lidar data. Different interpretations of common terms, competing claims among stakeholders, along with confusion between theoretical or operational accuracy specifications also contribute to the lack of acceptance and skepticism regarding lidar accuracy claims. The ASPRS LiDAR Subcommittee is working hard to establish guidelines for calibrating lidar systems and providing commonly accepted definitions of lidar accuracy in an effort to standardize these specifications. (Continued on page 6)
B r ie f ing N o te
More 0.50 0.40 0.30 0.20 0.10 0.00 -0.10 -0.20
4.
-0.30
3.
Norm alized to 402 Cnt Pts
1. Manufacturer’s accuracy specifications are derived from statistical sampling of the lidar data and are generally quoted as a 1 sigma spec, meaning ~68% of the data will fall within
2.
this limit. 2 sigma (95%) or 90% (1.6 sigma) specifications are generally not mentioned. Accuracy specifications are generally taken across the entire scan width of a system despite the fact accuracy will decrease with increasing scan angle; it is common to see the quoted accuracy being the average of the error at min and max scan angles. Accuracy is generally taken in the GPS reference frame so effects of geoid modeling are ignored. Accuracy analysis is generally taken by comparing to known ground control points but details of how this was done are generally not included.
P roject #1 - E rror H istogram
A brief survey of accuracy specifications published current service providers and system manufacturers would reveal specifications of 15 cm vertical and 50 – 100 cm horizontal are common. The published specifications generally provide little information on how this accuracy is measured including such critical information as operating altitude, full or limited scan angle, target type and slope, GPS quality required to reproduce etc. Some notes to keep in mind when discussing lidar accuracy:
0.60
System Accuracy Specifications
(Continued on page 15)
Table 1
-0.50 Les s 0.000
0.050
0.100
0.150
0.250
0.300
0.200
Count (Normalized)
Min Max Mean Stdev RMS
A recent volumetric survey conducted using our ALTM 1225 was flown for 0.5 ft RMS accuracy. The -0.58 survey was flown at 2500 feet and 20,000,000 0.40 points were collected covering 5 sq. miles. -0.07 Ground control was based on 400 control points 0.16 and kinematic GPS profiles. The results are 0.17 shown in Table 1 which lists the min, max, mean and standard deviation of the lidar data from ground control (all units US Survey Feet).
-0.40
P R O J ECT S T U DY # 1
D eviation From C ontrol (U S S urvey Feet)
Page 1 Page 6
PPage 8
1.5
B r ie f ing N o te
-0 .9
(Continued on page 10)
Table 2
-1.3 Less
0.000
0.020
0.040
0.060
0.080
0.100
0.120
A recent preliminary engineering and design survey was flown for 0.5 RMS accuracy. The survey was flown at 3000 feet and 150,000,000 points were collected covering 40 sq. miles. Ground control was based on 4500 control points and kinematic GPS profiles. The results are shown in Table 2 which lists the min, max, mean and standard deviation of the lidar data from ground control (all units US Survey Feet).
-1.39 1.19 0.04 0.40 0.40
Count (Normalized)
Min Max Mean StDev RMS
-1.1
P R O J E C T S T U DY # 2
D eviation From C ontrol (U S Survey Feet)
0 .9 0 .7 0 .5 0 .3 -0 .7
-0 .5
-0 .3
-0 .1
0 .1
No r m aliz e d to 4428 Cnt Pts
In addition, atmospheric affects can impact the accuracy of the laser rangefinder and become significantly more critical at higher altitudes. Proper modeling of these affects is necessary to prevent introducing additional error in to the error budget. These atmospheric affects are wavelength-dependent so they can vary in magnitude depending on the particular wavelength used in the system. The correction for the refraction and velocity change of light in the atmosphere is given as a spherical range correction (Marini and Murray, 1973) and needs to be calculated at both the platform location
1.1
position on transmitted and received pulses or range walk due to spreading of the pulse from a sloped target.
P roject #2 - Error H istogram
Laser rangefinders are mature technology that is reasonably well understood. The engineering requirements, system design parameters and performance envelopes achievable with current technology are well known. The majority of today’s airborne lidar systems are based on diode-pumped solidstate lasers with pulse widths on the order of 10 ns and rise times on the order of 1 ns. Under normal operating conditions, the range error from a properly calibrated laser rangefinder of this caliber can be expected to be on the order of 5 – 7 cm, independent of altitude. However, proper calibration of the rangefinder requires the system engineer to take in to consideration such parameters as the timing jitter in the on-board clock (time interval meter), the ability to select the same relative
1.3
L A S ER R A NG EF IN D ER ERROR
PPage 10
B r ie f ing N o te
L ASER RA NGEFIND ER ERROR (Continued from page 8)
and the laser footprint. The magnitude of the atmospheric correction is dependent on temperature, pressure, and humidity and to some degree on the altitude above sea level and the latitude. These atmospheric affects are usually minimized but not eliminated - by incorporating an appropriate atmospheric model in the postprocessing of the lidar data.
“In general, nonuniform targets with differences in reflectivity and significant slope across the footprint introduce uncertainty in to the actual XY position being referenced by the return pulse with a corresponding uncertainty in Z position.”
If proper system design considerations are implemented, the laser rangefinder accuracy is the best-defined and smallest contributor to the overall error budget except in cases of low altitude, low scan widths. An excellent discussion on geolocation of laser altimetry data can be found in Hoften et. al. (2000). A Note on Divergence and Footprint Size The divergence of a laser represent the physical spread of the beam as it propagates. For example an output beam with a spot size of 0.1 cm and a divergence of 0.25 mrad as it exits the sensor will illuminate a footprint on the ground of ~25 cm from an altitude of 1000 m and ~50 cm from 2000 m. Divergence is a physical property of the laser source that can be modified incorporating appropriate optical elements in the transmitter. Typical divergence values for commercial sensors range from 0.25 mrad to 5 mrad.
For small footprint, time-offlight lidar sensors, the footprint size is an important consideration when considering accuracy. Lidar sensors are not infinite point sampling instruments and the complex interaction of the transmitted pulse energy with the target needs to be considered carefully. The return signal from a target surface will be a function of the integrated energy distribution across the footprint weighted by the reflectivity profile of the terrain within the footprint. Increasing slope across the footprint can further complicate this situation compared to a flat target, as can returns where only part of the footprint intersects the target. In general, nonuniform targets with differences in reflectivity and significant slope across the footprint introduce uncertainty in to the actual XY position being referenced by the return pulse with a corresponding uncertainty in Z position. This situation can be minimized by using time-of-flight systems with the smallest divergence possible, resulting in the smallest footprint on the ground.
Page 11
G P S P OS I T I ON I N G ER RO R Use of the Global Positioning System (GPS) is an important part of lidar mapping. Airborne GPS systems are used in lidar to provide positioning information regarding the trajectory of the sensor. When performing lidar mapping, it is important to have a good understanding of GPSrelated errors. GPS errors fall into two broad categories. The first category is carrier phase GPS positioning errors. Differential carrier phase positioning gives centimeter-level accuracy and differential code positioning gives meter-level accuracy. Sources of error include satellite geometry (PDOP), the number of satellites, orbital biases, multipath, antenna phase center modeling, integer resolution, and atmospheric errors. Atmospheric errors consist of either tropospheric or ionospheric errors. Compounding some of these errors is the distance from the ground GPS stations to the lidar sensor located in the aircraft. Another area of GPS-related errors falls in the category of network or ground control surveys. These error sources all have an impact on the accuracy of lidar products. One example of GPS-related errors is the accuracy of geoid height models. GPS heights are relative to an
ellipsoid, or mathematical figure of the earth. Most engineering and mapping projects are referenced to an orthometric height surface (i. e. elevations), so GPS users requiring orthometric heights need to perform geoid modeling. Empirical error estimates using current geoid height models produced by NGS show large differences using single-tie geoid modeling techniques. Nationally, the current geoid height model (GEOID99) has a precision of ±5.2 cm (1s) at a 5km distance, ±5.5 cm (1s) at a 10-km distance, and ±6 cm (1s) at a 100-km distance (Smith 2000). Any vertical GPS error, such as geoid height modeling, will directly influence the accuracy of any lidar product. It is important to remember that for every GPS-related error source, a method can be employed to detect, eliminate, or minimize that error. This generally involves including the right kind of checks into the process to detect these data outliers. One of the checks performed by Airborne 1 includes a statistical comparison of lidarderived DEMs against kinematic GPS profiles performed by field surveyors.
“Empirical error estimates using current geoid height models produced by NGS show large differences using single-tie geoid modeling techniques.”
0 .3 5
B r ie f ing N o te
0 .2 0 0 .15 0 .10 0 .0 5 0 .0 0 -0 .0 5 -0 .10 -0 .15
(Continued on page 15)
Nor m aliz e d to 88 Cnt Pts
A 0.005º angular error corresponds to a 0.17 m positioning error on the ground from 2,000 m, a 0.35 m error from 4,000 m and a 0.52 m error from 6,000 m. However there are additional contributions to the angular pointing error that are generally not discussed by sensor manufacturers or service providers. These include contributions from the scanning subsystem, which will add to the error budget due to the finite accuracy in measuring the scanner mirror angle and non-linear dynamics
0 .2 5
(POS/AV™ 510 from Applanix – postprocessed solution) although some systems perform to a 0.0025º pitch/roll accuracy.
Proje ct #3 - Error Histogram
Knowing the correct orientation of the sensor in space is a necessary but not sufficient condition for calculating an accurate transformation from the local sensor reference frame to the Earth-centered reference frame (WGS84). Accurate measurements of the roll, pitch and heading of the platform are required to correctly determine the pointing direction for each laser pulse. In practice, the orientation of the platform is recorded by an on-board inertial measurement unit (IMU) that is hard mounted to the lidar sensor. While a variety of IMUs are available commercially, a typical specification for the price/performance levels common in most commercial lidar sensors would be 0.005º pitch/roll, 0.008º heading
0 .3 0
I MU O R I EN TA T IO N E RRO R
-0 .2 5 -0 .3 0 -0 .3 5
0.000
0.050
0.100
0.150
0.200
0.250
0.300
Table 3
Count (Normalized)
Min Max Mean StDev RMS
A recent high resolution DTM survey for a Hollywood studio was flown for highest density, 0.5 RMS accuracy. The survey was flown at 2000 feet and -0.40 over 150,000,000 points were collected cov0.24 ering 10 sq. miles. Ground control was based 0.01 on 90 points and kinematic GPS profiles. 0.10 The results are shown in Table 3 which lists 0.10 the min, max, mean and standard deviation of the lidar data from ground control (all units in US Survey Feet).
-0 .2 0
P R O J E C T S T U DY # 3
De v iation From Control (US Surv e y Fe e t)
PPage 12
Page 1
Page 15
IMU Orientation Error (Continued from page12)
De v iation From Control (US Surv e y Fe e t)
-0.90 -0.80 -0.70 -0.60 -0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 More
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
0.1600
0.1800
No r m aliz e d to 89 Cnt Pts
Count (Normalized)
Project #4 - Error Histogram
L id ar Acc ur a c y
namics in the scanner mirror motion, especially for single-axis as opposed to rotating scanner systems, and laser pointing errors. Many of these effects can be minimized but not eliminated - by proper system calibration prior to data collection and proper system modeling during post-processing. However, it is important to keep in mind that the angular error budget – the orientation error – is not just derived
from the IMU but rather is the sum of its components. A sensor with a perfect IMU providing absolute accuracy measurements of the platform orientation would be wasted if the scanner subsystem only had an accuracy of 0.5º. In general, when considering final achievable accuracy for the lidar sensor, system engineers must consider the entire error budget and not simply focus on the IMU.
Accuracy Specifications (Continued from page 6)
5.
Accuracy analysis tends to focus on vertical accuracy (Z) and details on how planimetric accuracy (XY) is verified are vague.
It is always prudent to request a certified system calibration and accuracy analysis from the lidar system manufacturer or the service provider.
P R O J E C T S T U DY # 4 Another recent preliminary engineering and design survey was flown for 0.5 RMS accuracy. The survey was flown at 3000 feet and over 50,000,000 points were collected covering 5 sq. miles. Min Max Ground control was based on 90 control points and kinematic GPS profiles. The re- Mean StDev sults are shown in Table 4 which lists the RMS min, max, mean and standard deviation of the lidar data from ground control (all units in US Survey Feet).
-0.85 0.54 -0.02 0.29 0.29 Table 4
PPage 16
B r ie f ing N o te
MEA SU R IN G U P S TA N D A R D S Q: “Can you do 1’ contourinterval mapping with your laser?” The question comes across our phone lines with everincreasing frequency. As with most every lidar service provider, our answer remains unchanged. The problem is, the answers vary significantly across companies.
“Can you do 1’ contourinterval mapping with your laser?”
As a relatively new tool, it is important to remember that lidar represents a different measurement system when compared to photogrammetry. Current mapping standards were developed with classical photogrammetric systems in mind. Certain artifacts can occur with lidar-derived products such as gradebreak definition, planimetric accuracy issues, and vegetation removal. It is important for lidar users to understand when these artifacts occur so that mapping products meet the desired tolerance for the project at hand. To date, there is a lack of specifications or standards for lidar products. Because of the lack of lidar standards and specifications, one must use existing guidelines to estimate the accuracy of lidar products. At Airborne 1 we are committed to supporting ASPRS’s effort to establish and publish
TO THE
guidelines and standards to address these issues. However, with enough effort and thought, there’s usually a much more helpful answer than “it depends.” If this booklet achieves its design, it will illustrate and substantiate our response to this question, which says: A: “For the vast majority of terrain and vegetation types, our lidar DEM data will support 1’ c.i. mapping which meets or exceeds the 1’ NMAS accuracy requirements, and of course those of any larger contour interval. We will not certify data to the more rigorous 1’ c.i. requirements defined by ASPRS Class 1 map accuracy standards. Given the capability of today’s lidar technology, we will also reliably and repeatedly certify to ASPRS Class 1 standards for a 2’ c.i. map product. At the margin, we find most datasets will in fact support certification at up to ½ meter contours according to the more rigorous standards.” The support for that position comes directly from the statistical analysis on the millions of acres and thousands of flight-lines of lidar data we’ve collected. In this booklet, we’ve posted an analysis (Continued on page 17)
L id ar Acc ur ac y
Page 1
(Continued from page 16)
chart for each of our five most recent projects. It is our hope that sharing some of that analysis will be at least informative, and may open doors to exchanges of information across providers and users alike. Here’s one example of how our recent project statistics measure up against the applicable NMAS, ASPRS, and NSSDA standards. Project: Fullerton, CA Deliverable: DEM in support of 1’ contours to NMAS standards Control Points/Total Control Points: 403/407 Lidar DEM: TIN values against control points: Vertical in Feet
Feet -0.577 0.403 -0.067 0.159 0.172
Min Max Mean Stdev RMS Table 5
Applying Mapping Standards to LiDAR NMAS: Vertical requirements are that at least 90%
Page 17
of tested elevations be within ½ the contour interval, or in this case 0.5 feet. For this project, the RMS value is 0.172 feet. This equates to 0.283 feet at the 90% level of confidence. This was calculated by simply multiplying the RMS error x 1.6449. Clearly, the data meets and exceeds the vertical requirements of the NMAS standard, assuming the distribution is normal. Horizontally, our ALTM 1225 laser unit reliably and repeatedly demonstrates better than 1/5000th the flight altitude, including all calibration and test data provided by the manufacturer, Optech. Their systems are typically certified to better than 1/1000th, 1/2000th , or some higher (“better”) number if applicable and proven to a specific system. Importantly, all these statements are for 1-sigma (68% +/-) only. The 1225 unit we’re flying has been thoroughly tested and proven at several times these ratios, but still meets or exceeds the 90% test at 1/30th and/or 1/40th of an inch at the publication scale for 1”:40’ map products (or 1.33’ and 1.0 feet, respectively). This is conservatively calculated at 1/3,500th the flight altitude as follows: 2000’ (flight altitude) divided by 1:3500 horizontal accuracy factor yields 0.5714 feet at 1sigma. Multiply this (Continued on page 18)
“At Airborne 1 we are committed to supporting ASPRS’s effort to establish and publish guidelines and standards to address these issues.”
PPage 18
B r ie f ing N o te
MEA SU R IN G U P S TA N D A R D S (Continued from page 17)
times the factor of 1.6449 (to ascertain the 90% confidence level), and he resultant accuracy is 0.940 feet. This falls inside the frequently cited requirement of 1 contour interval, and exceeds the 1/30th of an inch requirement at 40-scale. ASPRS: Class 1 map vertical accuracy requirements are for a limiting RMS error of 1/3rd the contour interval. The sample project meets this vertical standard, with an RMS error of 0.172 feet for the mass points. Notably, the data fails to meet the spot elevation accuracy requirement of 1/6th the contour interval (0.167’), so these would have to be compiled from imagery or ground collection methods. Horizontally, though, the limiting RMS error for a 40scale map (1”:40’) is .40
TO THE
feet. For a system flying at 2000’ AGL, and carrying a “1/2000th the altitude” horizontal certification, the expected planimetric accuracy is going to be 1.0’, clearly falling outside the specification. Because our horizontal is tested to something better than this, we can plan and fly to generally achieve a 0.4’ horizontal requirement on certain projects. We cannot yet do so reliably, repeatedly, and certifiably, and therefore will not certify datasets to this standard at this time. For 100-scale map products, this horizontal requirement is eased to 1.0’, and therefore is readily certified to. NSSDA: This standard relies on accuracies reported at the 95% level of confidence, a geometrically more (Continued on page 19)
NOTE ? P LAN I M E TR I C VERTICAL ACCURACY The accuracy specifications for planimetric (XY) accuracy as opposed to vertical (Z) accuracy are different for lidar data. The planimetric accuracy is strongly correlated to the pointing angle accuracy and due to the angular error component increases with altitude. Unlike
V S.
imagery, planimetric accuracy for lidar data is generally 2 –5 times worse than vertical accuracy.
L id ar Acc ur ac y
Page 1
MEA SU R IN G U P S TA N DA R D S challenging specification to deliver on. In our example, the RMS error of 0.172’ times the required 1.96 factor yield an accuracy of 0.337’ at the 95% level of confidence. The vertical requirement of the NSSDA standard states that the data must be better than 0.5958 x the contour interval at the 95% level of confidence. The sample project meets this vertical standard.
Page 19
TO THE
which yields a horizontal error of 0.421 feet on this data example.
Lidar DSM of San Francisco
For horizontal accuracies, this standard again seeks the results at a 95% level of confidence. As with all these tests, a normal distribution is assumed. Assuming independent error in the x and y axis, a factor of 2.4477 is used to determine the horizontal NSSDA accuracy,
P R O J E CT S T U DY # 5 A recent DEM survey in support of photogrammetry was flown for 0.5 RMS accuracy. The survey was flown at 3000 feet and over Min 100,000,000 points were collected covering Max 10 sq. miles. Ground control was based on Mean 150 control points and kinematic GPS pro- St Dev files. The results are shown in Table 5 RMS which lists the min, max, mean and standard deviation of the lidar data from ground control (all units in US Survey Feet).
-0.40 0.35 -0.02 0.14 0.15 Table 6
More
L id ar Acc ur ac y
0.50
ACCURACY SUMMA RY FO R L AS T 5 AI RB OR NE 1 P ROJ EC TS In summary, the vertical accuracy we are seeing from our Optech ALTM 1225 based on our last five field projects is as follows:
Deviation From Control (US Survey Feet)
0.40 0.30 0.20 0.000
0.050
0.100
0.150
0.200
0.250
0.300
-0.50
-0.40
-0.30
-0.20
-0.10
0.00
0.10
Normalize d to 144 Cnt Pts
Count (Normalized)
Project #5 - Error Histogram
Page 1
Project
Goal (RMS, feet)
RMS (feet)
#1
0.50
0.17
#2
0.50
0.40
#3
0.50
0.10
#4
0.50
0.29
#5
0.50
0.15
Avg
0.22
Table 7
REF ERENCES Baltsavias, E.P. (1999). Airborne laser scanning: basic relations and formulas. ISPRS J. Photogramm. Remote Sensing 54 (2/3), 199-214. Gutierrez, R. et. al. (1998). Airborne Laser Swath Mapping Of Galveston Island And Bolivar Peninsula, Texas, in Proceedings, Fifth International Conference: Remote Sensing for Marine and Coastal Environments, San Diego: p. I-236–i-243. Hoften, M.A. et. al. (2000). An airborne scanning laser altimetry survey of Long Valley, California. Int. J. Remote Sensing, Vol 21, #12, 2413-2437 Kraus, K., Pfeifer, N., (1998). Determination of terrain models in wooded areas with airborne laser scanning data. ISPRS J. Photogramm. Remote sensing 53 (4), 193-203. Marini, J.W. and Murray, C.W. (1973). Correction of laser range tracking data for atmospheric refraction at elevation angles above 10º. X59173351 NASA Technical Report Shrestha, R.L. et. al. (2000). Airborne Laser Swath Mapping: Accuracy Assessment For Surveying and Mapping Applications. University of Florida (see http://www.alsm.ufl. edu/pubs/accuracy/accuracy.htm) Smith, D.A. (2000). Gravity and the Geoid at NGS. Presented at the 2000 Geodetic Advisor Convocation. National Geodetic Survey, Silver Spring, MD
Page 21
205.00 KByte
5.08 GByte
20.50 MByte
13.16 GByte
The Appendix includes FEMA's requirements for LIDAR systems to gather the necessary data to create digital elevation models, digital terrain maps and other National Flood Insurance Program products. FEMA is working with the National Digital Elevation Program and coordinating with the American Society of Photogrammetry and Remote Sensing to support the develop of broad government and industry standards for LIDAR and other advanced remote sensing technologies. As these standards are adopted, FEMA intends to replace the current FEMA standard with these broader standards. This guidelines are available at http://www.fema.gov/mit/tsd/dl_cgs.htm.
Per Acre
100 acre
N OTE S:
100 sq. km
25,000 Hz x 3,600 s x 52 byte = 4.57 GByte
FEMA P UBLISHES A PPENDIX FOR L I DAR As part of its Map Modernization Plan, FEMA has developed a specification for the production of elevation data for flood studies using LiDAR systems. This has been incorporated into Appendix A of the new Guidelines and Specifications for Flood Hazard Mapping Partners.
100 sq. mile
Intensity Data Size – Dual Return wt Intensity
XYZ Co-ordinate
Intensity
Processed Data – Dual Return wt Intensity 3 XYZ Co-ordinate
Time Stamp
Scan Angle
Laser Range
Intensity
Data Size - Dual Return wt Intensity
Raw Data – Dual Return wt Intensity Laser Range Intensity
Parameter
Data Storage Requirements (25,000 Hz, dual return with intensity) Per Hour of Data Collection (“Button On Time”) Per Project Area (assuming 1 pulse per 10 feet2 (~ 1 m2))
2 byte 52 byte
2 byte
3 x 8 byte = 24 byte
3 x 8 byte = 24 byte
8 byte
22 byte
2 byte
2 byte
4 byte
4 byte 2 byte Data Size (typical)
REMENTS
LI DAR DAT A ST ORAGE REQUI
L id ar Acc ur ac y
Page 1
Page 23