PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
July 2019
469
Question:
I am a college student working on my bachelor’s degree
in Spatial Science (Surveying).
I am interested in photogrammetry, and my study is on drone-
based surveys. I have the following questions on the new “ASPRS
Positional Accuracy Standards for Digital Geospatial Data”:
1. In sections 7.7 and 7.8, when it talks about checkpoint and
ground control accuracy being 1/2 RMSEmap, is RMSEmap
the desired/intended accuracy class?
2. With GCPs having three times the accuracy of the geospatial
data set being tested, does that mean the GCP accuracy will
be three times more accurate than the desired/intended
accuracy class?
3. Do you use Table D.1 to calculate all the statistics and then use
the results to determine the ASPRS accuracy class? Is that the
typical workflow? Is there a sample report you can supply?
4. Are there guidelines on what you should aim for regarding
the additional statistics discussed on the standards, such as
max, min, skew, kurtosis and mean absolute?
5. Can you clarify what it means when you can state “tested to
meet” versus “produced to meet”?
6. Can you direct me to a document regarding planning and best
practice guidelines?
7. Do the vegetated area ground control points simply go on the
bare ground between vegetation?
8. How do you assess seamline mismatch?
9. If an orthophoto fails a column in Table B.3 (e.g. the RMSEr is
OK, but the accuracy at 95%CI exceeds the limit), do you select
the accuracy class in which your project meets or exceeds
all standards in a single row?
10. I note that many drone-based surveys seem to have a mean
error much higher than 25% of the RMSE. What does this
information tell you about the quality of the project, and how
can you correct it?
11. I also noticed that nearly every drone software company
reports accuracy as a function of GSD, e.g. heights within
three times the GSD. How are these related, and is GSD really
related to accuracy in any way? I did a project with a GSD of
1 cm, but I achieved 11 mm RMSE heights and mean of 3 mm.
12. Can you direct me to where I can read more about rigorous
total propagated uncertainty regarding photogrammetry?
13. For repeat surveys of the same area, if I use the software to
determine the camera calibration via self or automatic calibra-
tion, is it best to save it and then use the same calibration for
the repeat flights rather than having the software re-calculate
the camera calibration each time?
James Wallace
University of Southern Queensland, Australia
Dr. Abdullah:
In the June 2019 issue, I addressed questions 1
through 6. The remaining questions are addressed below.
P
art
II
Question 7—
Do the vegetated area ground control
points simply go on the bare ground between vegeta-
tion?
Answer:
Yes, they should. Ground control points should
always be surveyed on a firm ground.
Question 8—
How do you assess seamline mismatch?
Answer:
Evaluating seamline mismatch is usually per-
formed visually to determine whether features in adja-
cent rectified images are aligned correctly. For standard
large-format metric cameras, mosaic cut lines are inserted
manually. The technician selects the mosaic cut lines path
to avoid buildings, trees and other elevated objects, as well
as radiometrically mismatched areas. Those mosaic cut
lines are usually saved in a CAD file format such as DGN,
DWG or shape file. These CAD files, if they exist, are very
Photogrammetric Engineering & Remote Sensing
Vol. 85, No. 7, July 2019, pp. 469–471.
0099-1112/19/469–471
© 2019 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.85.7.469
