PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
September 2016
661
Table 2 Examples on horizontal accuracy for digital orthoimagery interpreted from ASPRS 1990 legacy standard.
Common Orthoimagery Pixel Sizes
Associated Map Scale ASPRS 1990 Accuracy Class
Associated Horizontal Accuracy According to Legacy ASPRS 1990 Standard
RMSE
x
and RMSE
y
(cm)
RMSE
x
and RMSE
y
in terms of pixels
0.625 cm
1:50
1
1.3
2-pixels
2
2.5
4-pixels
3
3.8
6-pixels
1.25 cm
1:100
1
2.5
2-pixels
2
5.0
4-pixels
3
7.5
6-pixels
2.5 cm
1:200
1
5.0
2-pixels
2
10.0
4-pixels
3
15.0
6-pixels
5 cm
1:400
1
10.0
2-pixels
2
20.0
4-pixels
3
30.0
6-pixels
7.5 cm
1:600
1
15.0
2-pixels
2
30.0
4-pixels
3
45.0
6-pixels
10 cm
1:800
1
20.0
2-pixels
2
40.0
4-pixels
3
60.0
6-pixels
15 cm
1:1,200
1
30.0
2-pixels
2
60.0
4-pixels
3
90.0
6-pixels
30 cm
1:2,400
2
60.0
2-pixels
3
120.0
4-pixels
3
180.0
6-pixels
60 cm
1:4,800
1
120.0
2-pixels
2
240.0
4-pixels
3
360.0
6-pixels
of pixel size and map scale are based on what we developed
for scanning an aerial film using 21micron/pixel scanning
resolution. According to Table 2, imagery with a GSD of 10
centimeters is sufficient to produce amapwith a scale of 1:800
with three different accuracies, RMSE = 20 centimeters or
(2*pixel size) for class I, RMSE = 40 centimeters or (4*pixel
size) for class II, or RMSE = 60 centimeters or (6*pixel size)
for class III. In the new ASPRS standard, we are saying it
is up to the user to request what accuracy he or she wants
the products to be labeled as, without any reference to map
scale, GSD, or contour interval. However, expecting the
confusion that map users and producers may experience
during the early stage of the new standard implementation,
the new standard provided some guidelines and current best
practices. Table 3 provides general guidelines to determine
the appropriate orthoimagery accuracy class for three
different levels of geospatial accuracy. The new standard
states:
“
The orthoimagery pixel sizes and associated
RMSE
x
and RMSE
y
accuracy classes presented in
Table 3 are largely based on experience with current
sensor technologies and primarily apply to large
and medium format metric cameras. The table
is only provided as a guideline for users during
the transition period to the new standard. These
associations may change in the future as mapping
technologies continue to advance and evolve.”
“Digital imagery and other digital aerial products
should not be labeled or associated with map scale,
GSD, and/or contour interval”
