260
April 2017
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
vertical accuracy of the airborne GPS used to process the
imagery. However, with four ground control points, two at each
end of the 1.3-mile corridor, the horizontal accuracy and vertical
accuracies came down to the submeter level. Additional ground
control points along the corridor helped bring the horizontal
accuracy to under 0.20 feet and the vertical accuracy to under
0.30 feet. To assure vertical accuracy of RMSE = 0.25 feet or
better, it is recommended to have a pair of ground controls every
500 to 700 feet along the corridor.
C
onclusions
and
R
ecommendations
1.
The controlled experiments discussed in Cases I
and II clearly show that one can obtain vertical and
horizontal accuracy of 0.25 feet or better from UAS-
derived products with the proper mission planning and
ground control design. Better results may be possible
if the flying altitude is lowered and a different control
network is used.
2.
The latest actual corridor results using the Renaissance
system exceeded the recommended spacing of ground
controls stated in the conclusions for Case II. On a
recent corridor construction project, where we fly over
our client’s project every three months, our client’s team
of surveyors are finding that we are meeting a 0.20
feet vertical accuracy from 3cm imagery flown from an
altitude of 1,750 feet AGL. The average spacing between
pairs of ground control along the 19 miles corridor
was around 3,000 feet. This is a real testament on the
accuracy of the products derived from a UAS surrogate,
as the surveyors did an extensive field check using the
latest technique in field surveying multiple times.
3.
One needs to be aware of the accuracy requirements
for the ground control and checkpoints used in the
photogrammetric workflow to produce orthorectified
imagery and digital elevation model from UAS-based
imagery. The new ASPRS mapping standard, “ASPRS
Positional Accuracy Standards for Digital Geospatial
Data,” calls for the accuracy of the ground control
points used to produce any products through the
photogrammetric process to be always twice as good
as the accuracy expected for the generated products.
Therefore, the accuracy of the ground control used in
the aerial triangulation process should be two times
more accurate than the expected accuracy for aerial
triangulation. In the same token, the accuracy of the
aerial triangulation should be two times better than the
accuracy of the orthos and/or the digital elevation model
produced using the triangulated imagery. In other words,
according to the ASPRS Positional Accuracy Standards
for Digital Geospatial Data, the following accuracy
figures/relationship needs to be satisfied:
“Ground control points used for aerial triangulation
should have higher accuracy than the expected accuracy
of derived products according to the following two
categories:
•
Accuracy of ground control designed for planimetric
data (orthoimagery and/or digital planimetric map)
production only:
RMSE
x
or RMSE
y
= 1/4 * RMSE
x
(Map)
or RMSE
y
(Map)
,
RMSE
z
= 1/2 * RMSE
x (Map)
or RMSE
y
(Map)
•
Accuracy of ground control designed for elevation data,
or planimetric data and elevation data production:
RMSE
x
, RMSE
y
or RMSE
z
= 1/4 * RMSE
x
(Map)
, RMSE
y
(Map)
or RMSE
z
(DEM)
4.
It is important to understand the above accuracy
requirements to challenge people who claim that they
can meet subcentimeter accuracy from UAS. In order
for them to meet a 1cm vertical accuracy, their ground
control should be surveyed to an accuracy of 0.25cm
or better, according to the ASPRS Positional Accuracy
Standards for Digital Geospatial Data. Such tight
accuracy is hard if not impossible to meet using current
GPS-based surveying techniques.
5.
All results discussed in Cases I and II are based on
standard consumer-grade GPS with accuracy of 1.0 to
2.0 meters. Using RTK-based GPS for UAS operations
shall definitely result in an improvement in the accuracy
of the derived products.
Finally, going back to your question and as you see from my
case studies, it is difficult to predict any accuracy figures
for your products without knowing the operational details
surrounding the mission circumstances that may affect the
accuracy of the derived products. As for your question on
whether a ratio-based relationship exists between horizontal
and vertical accuracy, I did not noticed any other than
that the horizontal accuracy is stabilized with less ground
control points than the number needed to bring the vertical
accuracy to a reasonable range. I hope the examples and the
recommendations I have provided here will provide some
guidance for you and other readers when dealing with UAS-
derived products in the future.
**Dr. Abdullah is Senior Geospatial Scientist and Associate at
Woolpert, Inc. He is ASPRS fellow and the 2010 recipient of the
ASPRS Photogrammetric Fairchild Award..
The contents of this column reflect the views of the author,
who is responsible for the facts and accuracy of the data pre-
sented herein. The contents do not necessarily reflect the offi-
cial views or policies of the American Society for Photogram-
metry and Remote Sensing and/or Woolpert, Inc.
