PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
November 2014
1019
Our operation utilizes an UltracamFalconPrime camera
with a lens focal length of 100mmand 6 microns pixel size.
I would appreciate it if you elaborate on the following:
1. What is the maximum image ground sampling dis-
tance (GSD) I can use to produce maps with a scale
of 1:480 (1”=40’) or 1:600 (1”=50’) and elevation data
(mass points and break lines) using softcopy com-
pilation to meet class 1 vertical accuracy according
to the American Society of Photogrammetry and
Remote Sensing (ASPRS) Map Accuracy Standard?
2. What is the maximum image GSD I can use to pro-
duce a map with a scale of 1:1,200 (1”=100’) using
softcopy compilation to meet class 1 horizontal
accuracy according to the ASPRS Map Accuracy
Standard for that scale?
3. How do I estimate the vertical accuracy based on
the GSD of the imagery?
Anonymous
Dr. Abdullah:
Your question presents common concerns among the mapping
community since we transitioned from using film-based
cameras to digital. The topics of accuracy,
flying altitude, and map scale were easier
to handle with film camera. The two main
large format brands of film cameras, Leica
and Zeiss, which dominated the market
then, were manufactured based on similar
philosophies. Both cameras were designed
to use a standard aerial film with a square
format of around 228 mm (or 9 in) and
both equipped with a lens with focal length
of around 152 mm (or 6 in). Such fixed
geometry made it easy to estimate product
accuracy based on map scale and flying
altitude. To understand where we are
now concerning the relationship between
image GSD derived from digital cameras
and the expected product accuracy, we
must visit the historical evolution of
Table 1 Relationship between film scale and derived maps scale.
Common Photography Scales (with 9” film format camera and 6” lens)
Scanning Resolution
(um)
Photo
Scale
1” = 300’
1” = 600’
1” = 1200’
1” = 2400’
1:3,600
1:7,200
1:14,400
1:28,800
Flying
Altitude
1,800‘ / 550 m 3,600’ / 1,100 m 7,200’ / 2,200 m 14,400’ / 4,400 m
Appox.
Ground
Sampling
Distance
(GSD) of
Scan
0.25’ / 7.5 cm 0.50’ / 0.15 m 1.0’ / 0.3 m
2.0’ / 0.6 m
21
Supported Map/ortho Scales and Contour Intervals
GSD 3” / 7.5 cm 6” / 15 cm 1.0’ / 30 cm 2.0’ / 60 cm
C.I.
1.0’ / 30 cm 2.0’ / 60 cm 4’ / 1.2 m
8’ / 2.4 m
Map Scale
1” = 50’
1” = 100’
1” = 200’
1” = 400’
1:600
1:1,200
1:2,400
1:4,800
aerial photography from film-based media to digital. When we
transitioned from analytical and optical-mechanical plotters
to softcopy photogrammetry, we needed to scan the film to
convert it to a digital format suitable for processing in the
softcopy photogrammetric station. At that time, most film
scanners were designed with scanning resolution of multiples
of 7 microns (7, 14, 21, etc.), and different users used different
scanning resolution. For all the practical reasons at the time,
a scanning resolution of 21 micron was widely use, and it
became the standard scanning resolution for a long time. Some
users, though, adopted 14 microns for their standard scanning
practice. Table 1 relates the resulting GSD of the scanned film
and the supported map scale and contour interval derived from
film-based cameras at different flying altitudes. The values in
Table 1 are derived based on the commonly used film-to-map
enlargement ratio of 6 and a c-factor of 1800.
Values presented in Table 1 were endorsed and widely
used by both map users and data providers during and after
the transition period from film to the softcopy environment.
Furthermore, the mapping community continued using the
values presented in table 1 even after the introduction of
the first digital camera at around the beginning of the last
decade. Although such association of the image GSD from
digital camera and the map scale and contour interval proved
to be a safe practice as the geometry (lens focal length and
