The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
185 
achieves 150 kHz pulse rates at up to approximately 1200 m 
AGL. However, in order to minimize the effect of terrain 
reducing swath covered, it was decided to fly at a slightly 
higher altitude, where the maximum pulse rate was typically 
110-120 kHz. Although designed to collect up to 4 returns 
from each outbound laser pulse (first, second, third and last), 
the nature of the vegetation cover typically resulted in a single 
or double return only. 
2.6 Mission Planning and System Settings 
Mission planning was accomplished using Leica Geosystems 
FPES software with an a priori DEM input. The use of an a 
priori DEM helps to minimize the number of flight lines by 
allowing adjustment of flight elevations and/or direction and 
reducing the total terrain relief over any given flight line. In the 
prairie areas, where there is little terrain relief and reliable 
DEM information is available, planning could be fully 
automated. 
In some areas, the accuracy or resolution of the available a 
priori DEM data was in question. This was particularly true in 
more mountainous regions. Therefore, in many cases, some 
manual planning was done to allow grouping of flight lines 
together a one altitude, while adjacent groups of flight lines 
could be flown at a higher or lower altitude as needed. These 
“altitude groups” could then be submitted for automated flight 
line layout. 
This process also minimizes the number of different altitudes to 
be flown in each flight, saving the time required for altitude 
changes. Furthermore, some of the mountainous areas had to 
be flown from altitudes as high as 19,000 feet AMSL. The 
twin-engine piston-propeller aircraft used on this project can 
take a full hour to reach that altitude. The planning process 
used minimizes the number of high-altitude flights required and 
also helps to minimize the duration for which the flight crew 
would require supplemental oxygen. 
2.6.1 Typical flight line layout: The remote nature of much 
of the area flown dictated a unique approach to project layout. 
The overall project is broken down into 80 km x 80 km blocks. 
Each block consists of a number of parallel 80-km-long flight 
lines, oriented in a north-south direction. In some unusual cases 
flight lines might be oriented parallel to geographic features, 
but most are oriented north-south. 
An east-west cross strip is flown over the north and south ends 
of the block, extending slightly beyond the ends and into the 
adjacent blocks. In this manner, a single cross-strip comes into 
contact with up to 6 blocks, thus minimizing the number of 
cross strips required. Within each cross strip, there are a 
minimum of two ground control points. These ground control 
points are used both as an accuracy check as well as for setting 
the XYZ reference. 
In addition to the flight lines and cross strips, a “cloverleaf” 
boresight flight pattern was performed at the beginning of each 
flight. This additional data is used later for confirmation and/or 
adjustment of IMU boresight calibration. 
2.6.2 Typical flight profile and LIDAR system settings: 
Typical missions were flown at 1600 m AGL with a speed over 
ground of approximately 130 knots (~67 m/s). This results in a 
typical “on-line” duration of 20 minutes to cover the 80 km line 
length. It is important to note that 20 minutes is the maximum 
straight-line flight duration recommended to prevent 
accumulation of IMU drift beyond the specified accuracy of the 
GNSS/IMU subsystem. The Leica IPAS CUS6 IMU was 
chosen for all LIDAR systems used on this project in order to 
minimize drift and maximize accuracy. 
At this nominal flying height, turbulence was typically minimal 
over the prairie areas. However, turbulence was sometimes 
excessive over the more mountainous areas. At the higher 
absolute elevations required over mountainous terrain (which 
are near the service ceiling of the aircraft), aircraft handling is 
less precise. This resulted in postponement of some acquisition 
areas pending better flight conditions. 
System settings varied slightly across the project area, 
depending on the amount of terrain relief within the flight lines. 
In general, flight plans put the system near the lower end of the 
MPiA operating envelope for a particular pulse rate. This was 
done to maximize accuracy. Typical pulse rates were between 
110 and 120 kHz, though sometimes as low as 100 kHz if large 
terrain relief was expected. Field of view was typically planned 
for 35-40 degrees, maximizing canopy penetration in any 
forested areas. 
Typical side overlap was 20% of the raw swath width, and all 
systems used employed active roll compensation. This 
provided more than adequate overlap, given the typical 30-40 
meter accuracy with which the pilot can navigate the planned 
flight line, and leaving additional margin for aircraft yaw due to 
crosswinds. 
The system settings and resulting net swath width under 
varying terrain relief is shown below in Table 1 for both MPiA 
operation as well as conventional lPiA operation. In all cases, 
liberal allowances for flight navigation tolerances were used; 
+/-50 m in the vertical direction and +/-70 meters in the 
horizontal direction. A maximum FOV of 40 degrees was 
specified in order to maximize foliage penetration and minimize 
height error at the FOV edge. Flying heights were adjusted in 
each rriode (MPiA or lPiA) so that the same point spacing 
could be obtained in both operating modes. 
At the planned flying heights for the project, the maximum 
pulse rate attainable in 1 Pi A mode is not vastly lower than that 
of MPiA operation. However, the real advantage is apparent as 
the amount of terrain relief within any flight line increases. As 
terrain elevation under the aircraft increases over parts of the 
flight line, the net swath width (after removing allowances for 
navigation error) decreases. The effect is greater when using 
lPiA operation. Furthermore, the effect is magnified as the 
amount of terrain relief increases. Therefore, it is clear that 
MPiA offers quantifiable advantages even beyond the 
theoretical 2:1 advantage provided under zero-terrain-relief 
conditions. 
Terrain relief (m) 
None 
300 
600 
Mode 
MPiA 
lPiA 
MPiA 
lPiA 
MPiA 
lPiA 
Pulse rate 
(kHz) 
120 
89.3 
120 
89.3 
120 
89.3 
Flight 
height (m 
above 
terrain) 
1600 
1100 
1300- 
1600 
800- 
1100 
1000- 
1600 
500- 
1100