2011 - Geoinformatics

Capture
to enable the ground directly below to be viewed. With the introduction
of digital photography nav-sights were no longer required
so the holes were closed up. By removing the internal and external
plates the resulting hole was ideally suited to accommodate the ALTM
sensor. Luckily, this was the most cost effective and quickest adaptation
for the plane, as minimal work was required to create the second
hole.
System Installations
With two holes now established, the next problem to be resolved
was how to get all the equipment in, powered up and leaving
enough room for the operator. Both systems can be operated by one
operator so there was no need to accommodate another person.
With both hatches situated on the right hand side of the plane there
remained plenty of room on the left hand side for both control racks.
And with the operator sitting between the two sensors it enables
them to operate the systems efficiently (see Figure 1).
Each sensor contains it own IMU, but the plane only has one GPS
antenna. Rather than add an additional antenna to the top of the
plane, a GPS Antenna Splitter (Diplexer) was fitted to feed GPS data
to both systems.
Flight Planning and Data Capture
When flight planning for dual capture several factors need to be
considered to ensure suitable data is collected. The primary factors
being the required point density from the LiDAR and the Ground
Sample Distance (GSD) of the imagery and their operational capabilities
need to be assessed to ensure usable data is collected from
Figure 2 - Clifton Suspension Bridge, Bristol captured as part of our MetroHEIGHT product range.
A r t i c l e
both sensors. The specification and operational capabilities of our
Optech ALTM 3033 means that the dual capture is flight planned to
optimise data from it.
Blom wanted to capture both 4cm GSD imagery and 1m post spacing
LiDAR and, to achieve this, planned to fly at 700m above
ground. At this height the imagery has a 60/40% overlap and the
LiDAR has a 20% overlap.
Several other factors need to be considered during flight planning.
Due to the additional weight the endurance of the plane reduced,
meaning shorter sorties have to be planned. Additional cross strips
need to be included to help with the calibration and matching of the
LiDAR data.
Although the planning is optimised for the LiDAR, the capture has to
be optimised for the quality of the imagery. This meant that sun
angles and cloud cover need to be assessed before any data is
acquired.
Data Processing
Once the data has been acquired, the processing flow lines follow
the standard processing procedures. The LiDAR is extracted to create
the point cloud; matching to ensure that overlapping flight lines
align with one another; classification to create a ground class. The
imagery is colour balanced; using the IMU/GPS data and base station
data an aerial triangulation is done; the images are mosaiced
into tiles; final QA and correction undertaken.
One benefit of dual capture is that a DTM can be created from the
LiDAR data and supplied for the imagery production to be used as
a surface model during rectification of the aerial photography.
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