PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
October 2019
705
Question:
I referenced your highlight article “Harnessing Drones
the Photogrammetric Way,” which you published in the May 2019
issue of PE&RS, in a conversation about how ground control points
(GCPs) are necessary to create accurate maps with unmanned
aircraft vehicles (UAVs). I am a licensed surveyor and strongly
agree that GCPs are necessary to create accurate maps. My
conversational opponent, if you will, insisted that GCPs are not
needed if you use an accurate, real-time kinematic (RTK) GPS
system on the UAV. Despite your years of research and experience,
he is unwilling to accept your assertion. In addition, in the journal
article on page 332, you mentioned that at least five GCPs are
required for an accurate map. Do you have other data that supports
the need for GCPs when mapping with UAVs?
Steve Moran, PLS, Moran Surveying Inc.
Dr. Abdullah:
Your colleague is correct if and ONLY if certain
conditions are met, and some of these conditions are difficult
to meet on a regular basis. These conditions include:
SYSTEM ELECTRONICS
The system electronics and events timing on board the un-
manned aircraft system (UAS) and its sensors needs to be
aligned to the degree so the shutter mid-exposure pulse
(MEP) is determined perfectly. There is always a time offset
between the GPS event stamp and the MEP. Some compa-
nies invest a great deal in determining this offset, so they
can correct for it during the post processing of the GPS tags.
They do this because, unless the offset is determined per-
fectly, the wrong camera position will be measured by the
on-board GPS. From my experience, very few UASs on the
market time this offset appropriately.
HEALTH OF GPS SIGNAL
The GPS constellation or position dilution of precision (PDOP)
supports accurate position determination. This requires fly-
ing at a certain time of day and changes according to the proj-
ect location, date and time. Very few UAS field operations
allow us to wait in the field until a better PDOP is met, as you
want to spend as little time as you can in the field. A bad GPS
signal can cause errors in the camera position. Without the
use of GCPs in the aerial triangulation to model shifts and
drifts, you will get positionally inaccurate orthos or digital
surface models (DSM). Also, along these lines, the GPS data
recording frequency has a noticeable impact on the accura-
cy of determining the UAS camera position. One hertz (Hz)
versus 20 Hz recording frequency can introduce substantial
positional errors during an unstable aerodynamic situation.
During the 1 Hz GPS recording, the UAS may move 6 to 8
meters between the two epochs. Depending on the flying con-
ditions and the method of interpolation used, estimating the
sensor position between two epochs can be challenging and
produce inaccuracies. The presence of GCPs in this case can
prevent the uncertainty of estimating the image’s perspective
center position.
GEODETIC DATUM CONVERSION
There is always the possibility of introducing an anomaly in
the datum conversion between GPS-based WGS84 and the
product’s intended vertical datum, such as NAVD88. By not
having any GCPs in the project area, you risk introducing er-
rors due to geiod modeling and conversion. Even when flying
with airborne GPS for manned aircraft operations, I never
advise anyone to execute a project without GCPs and to rely
only on airborne GPS. We have always used GCPs in every
project; it is an industry standard. Some of the newer UAS
operators neither understand nor appreciate that fact. For
that reason, industry experts are working hard to help them
“Having GCPs in the aerial triangulation
process provides peace of mind. With the proper
software, it can be used to model GPS shift and
inertial measurement unit (IMU) drift and can flag
datum compatibility problems”
Photogrammetric Engineering & Remote Sensing
Vol. 85, No. 10, October 2019, pp. 705–706.
0099-1112/19/705–706
© 2019 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.85.10.705
