PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
April 2017
255
“Very often, I listen to technical presentations at conferences
I attend and hear highly exaggerated claims about product
accuracy. You even hear the terms “subcentimeter” or
even “millimeter” absolute accuracy during some of these
presentations.”
Question: I would like to get your expert opinion on
a dataset I just received. It is UAS-based imagery
collected to produce a 50cm Digital Elevation Models
(DEM) and 5cm resolution true color orthos. I do not
have any other metadata related to the project. Is there
any way to help me guess the horizontal and vertical
accuracies of the generated products? Is there any
ratio-based relationship between the horizontal and
vertical accuracy of products generated from UAS?
Dr. Srini Dharmapuri, Michael Baker International
Pittsburgh (Beaver), PA
Dr. Abdullah:
Your question is very important to the community
of geospatial mapping as it comes at a critical time when
users, like you, are anxious and confused about the positional
accuracy of products generated from an unmanned aerial
system (UAS). Some UAS manufacturers are overselling
their products without having a thorough understanding and
appreciation for the topic of data positional accuracy. Very
often, I listen to technical presentations at conferences I’ve
attend and hear highly exaggerated claims about product
accuracy. You even hear the terms “subcentimeter” or
even “millimeter” absolute accuracy during some of these
presentations. I am not asserting that UAS-derived products
cannot be produced with high accuracy, but I am saying that
careful consideration needs to be taken when dealing with
UAS-based sensors. The payload on board any small UAS,
which forms the bulk of the UAS platforms utilized by the
geospatial community, is characterized by miniaturized
designs. Such reductions in size and weight of the payload
forces a painful reality for the manufacturers of these
small UAS’s, as they have to deal with miniaturized and
quality-compromised imaging and auxiliary sensors. Most
of the cameras offered in the market for UAS are consumer-
grade and cost a few hundred dollars. Similarly, the GPS
and inertial measurement unit (IMU) are characterized
by degraded performance and accuracy. If the user is not
educated enough on this reality, he or she may believe the
false accuracy claims made by some UAS manufacturers or
even data providers. Many precautions can minimize or even
overcome the shortcomings of the sensors on board a small
UAS. Things like starting with efficient flight planning to
result in sufficient overlap between the imagery, using RTK
or PPK-based GPS, providing a dense and accurate ground
control network, and using the right processing software,
to mention a few. All of these precautionary measures
taken during mission planning help assure high-quality,
highly accurate products. What makes this situation more
challenging is the absence of legitimate and independent
evaluation studies that users can trust to navigate their
way when it comes to UAS-derived product accuracy. I am
not aware of any governmental funding invested in the
independent evaluation of products derived from small UAS.
This bitter reality encourages me to share with you and other
readers my recent experience along these lines. Woolpert,
my employer, was one of the first geospatial and engineering
companies to invest in UAS, and was the first surveying and
aerial mapping company approved to fly a UAS commercially
in designated airspace, earning an FAA Section 333
Exemption. Like other users, our clients questioned us about
product accuracy. We too were in the dark about the accuracy
of the UAS-derived elevation data and orthos until last year
when we took a drastic measure to invest in an independent
review of the accuracy of products derived from small UAS. In
the next sections, I will discuss two case studies we conducted
to measure the accuracy of products derived from small UAS.
Photogrammetric Engineering & Remote Sensing
Vol. 83, No. 4, April 2017, pp. 255–260.
0099-1112/17/255–260
© 2017 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.83.4.255
