PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
August 2016
587
Q: For a project size of 5000 square miles, how many
checkpoints are needed to assess the accuracy of a
Lidar dataset? Please provide the breakdown for the
number of checkpoints according to the VVA and NVA
requirement per the new ASPRS Positional Accuracy
Standards for Digital Geospatial Data guidelines.
Dr. Srini Dharmapuri, Michael Baker International,
Moon Township, PA, USA
Dr. Abdullah:
Before I address the question, I would like
to bring readers’ attention to the latest map accuracy
standard used in the U.S. and probably around the world,
as it represents the first map accuracy standard designed for
digital mapping products and lidar-based digital elevation
data. In late 2014, the American Society of Photogrammetry
and Remote sensing (ASPRS) approved and later published
the new ASPRS Positional Accuracy Standards for Digital
Geospatial Data. The new standards can be found through
the following ASPRS website:
Going back to the question, the new standard states on page A20:
“Using metric units, ASPRS recommends 100 static
vertical checkpoints for the first 2,500 square kilometer
area within the project, which provides a statistically
defensible number of samples on which to base a valid
vertical accuracy assessment. For horizontal testing of
areas >2500
km
2
, clients should determine the number of
additional horizontal checkpoints, if any, based on criteria
such as resolution of imagery and extent of urbanization.
For vertical testing of areas >2,500
km
2
, add five
additional vertical checkpoints for each additional 500
km
2
area. Each additional set of five vertical checkpoints
for 500
km
2
would include three checkpoints for NVA and
two for VVA. The recommended number and distribution
of NVA and VVA checkpoints may vary depending on the
importance of different land cover categories and client
requirements.”
According to the above guidelines, we can estimate
the required number of checkpoints using the following
computations:
Number of checkpoints suggested for the first
2,500 km
2
(or 956 aq. miles)
= 100 (55 checkpoints to
be distributed within the non-vegetated vertical accuracy
(NVA) area and 45 checkpoints within the vegetated
vertical accuracy (VVA) area)
Additional five checkpoints
(3 checkpoints to be
distributed within the NVA area and 2 checkpoints within
the VVA area)
for each additional 500 km
2
(or 193 square
miles), or for a project area of 5,000 square miles:
Given project area =
5,000 square miles = 12,949 km
2
Area that needs additional check points =
12,949 km
2
– 2,500 km
2
= 10,449 km
2
Number of 500 km
2
lots =
10,449 km
2
/ 500 km
2
= 20.89
Number of additional checkpoints =
20.89 x 5 = 105
(63 checkpoints to be distributed within the NVA area and
42 checkpoints within the VVA area)
“Using metric units, ASPRS recommends 100 static
vertical checkpoints for the first 2,500 square
kilometer area within the project, which provides a
statistically defensible number of samples on which
to base a valid vertical accuracy assessment”
“The recommended number and distribution of NVA
and VVA checkpoints may vary depending on the
importance of different land cover categories and
client requirements.”
Photogrammetric Engineering & Remote Sensing
Vol. 82, No. 8, August 2016, pp. 587–591.
0099-1112/16/587–591
© 2016 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.82.8.587
