PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
June 2020
347
Question:
For one of our projects, we have been asked to provide
an orthorectified imagery product with a horizontal accuracy
of 10cm at 95% confidence, according to the ASPRS Positional
Accuracy Standards for Digital Geospatial Data of 2014. The
questions I have are the following:
1. What is the ground sampling distance (GSD) of the imagery we
need to collect and use to meet the required accuracy?
2. What is the accuracy of the ground control points we need for
the aerial triangulation?
3. What is the accuracy of aerial triangulation we need to meet?
4. Can we use RTK surveying techniques to survey the needed
ground control points?
Dr. Srini Dharmapuri CP, PMP, GISP, Sanborn
Dr. Abdullah:
First, I would like to reiterate the design phi-
losophy that the new ASPRS Positional Accuracy Standards
for Digital Geospatial Data of 2014 was based upon, and
that is that the standards are sensor agnostic and data
driven. With this philosophy, the standards do not endorse
the use of GSD, contour interval or map scale to express
product accuracy. Product accuracy should be determined
by user need and the fidelity and quality of the product
generation process. Certain imagery resolution can be used
to produce orthorectified products with different accuracies
based on the production process used, quality and number
of ground control points, and the quality and accuracy of
the digital elevation model used in the orthorectification
process. Another good reason for not associating product
accuracy with imagery resolution is the various designs of
today’s digital cameras. Film-based aerial cameras were
designed with one film format/size (9 inches or 229mm) and
one lens focal length (6 inches or 152mm), which enabled
us to predict product accuracy based on film scale or flying
altitude. Digital aerial cameras are made with a variety
of charge-coupled device (CCD) array size and lenses that
make it impossible to adopt one accuracy figure for all
of them based on the flying altitude or imagery ground
resolution. Table 1 illustrates this issue as it lists the flying
altitudes for six well-known digital cameras, all set to
acquire imagery with 15cm resolution. The table shows the
wide range of altitudes (1,440m to 3,538m above ground
level, or AGL) used for different cameras to acquire the
same 15cm imagery.
While products from these cameras are expected to meet the
highest accuracy when a stringent photogrammetric workflow
is followed, one needs to be extra careful when dealing with
imagery acquired from a very high altitude. Errors in the final
products caused by the residual errors in the computed exteri-
or-orientation parameters, especially the sensor attitudes (i.e.
omega, phi and kappa), are linearly proportional to the flying
altitude. Table 2 lists the degree of error expected in a product
Table 1. Digital cameras and flying altitude.
Sensor
Flying Altitude
(m)
Flying Altitude
(ft)
UltraCAM CONDOR (100mm)
3,261
10,698
ULTRACAM EAGLE MARK III
(92mm)
3,450
11,319
DMC III (92mm)
3,538
11,609
ADS100 (62.5mm)
1,875
6,152
PhaseONE 190MP
2,935
9,629
ADS80
1,440
4,725
Table 2. Relationship between flying altitude and product horizontal accuracy.
Flying Altitude AGL (ft)
Horizontal Error in X or Y (ft)
ft
meter
ft
cm
100.0
30.5
0.007
0.22
150.0
45.7
0.011
0.33
200.0
61.0
0.015
0.44
400.0
121.9
0.029
0.89
3,000.0
914.4
0.218
6.65
6,000.0
1,828.8
0.436
13.30
10,000.0
3,048.0
0.727
22.17
Photogrammetric Engineering & Remote Sensing
Vol. 86, No. 6, June 2020, pp. 347–349.
0099-1112/20/347–349
© 2020 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.86.6.347
