1020
November 2014
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
camera format) of most earlier digital cameras allowed a flying
altitude that is close to the flying altitudes given in Table 1,
caution should be exercised when it comes to some of the new
digital cameras that are equipped with lenses with very long
focal length. Longer focal length usually allows us to fly higher
and cover a wider swath of the ground; however, increased
flying altitude exaggerates the horizontal and vertical errors of
the derived products caused by errors in the determination of
the camera attitudes or orientations. More complications exist
with the new generation of digital cameras as there are so
many designs and configurations for the camera format (CCD
physical dimensions and CCD array size) and lenses. Despite
the disparity in the new camera designs, users continued the
use of the values provided in Table 1, as it remained difficult
to estimate the expected accuracy from each new sensor. It is
worth mentioning that the latest generation of digital metric
mapping cameras brought added quality to the mapping
process with enhanced optics quality, extended radiometric
resolution through a higher dynamic range, finer CCD
resolution, rigid body construction and precise electronics.
Such added camera quality coupled with the advancement in
the airborne GPS and the mathematical modeling within the
current photogrammetric processing software allowed us to
extend the limits on the flying altitude. One should bear inmind
that many of the photogrammetric rules that were followed in
the last six or seven decades were based on the capabilities
of legacy technologies and techniques utilized then in the
mapping process. Rules like film-to-map enlargement ratio
(usually with a value of 6) and c-factor (usually with a value
of 1,800 to 2,000) are based on the limitations of the optical-
mechanical photogrammetric plotters and the aerial films
resolution, and they are no longer valid due to the enhanced
quality of the new digital cameras and the production process.
Product accuracy is a result of many factors other than the
camera geometry and the lens focal length. Factors such as
the quality of camera calibration parameters, flying altitude,
amount of images overlap, quality of parallax determination
or photo measurements, quality of the GPS signal, quality
and density of ground controls, quality of aerial triangulation
solution, capability of the processing software to handle GPS
drift and shift and camera self-calibration, and the digital
terrain model used for the production of ortho photo are
examples of such additional factors.
Based on the previous discussions and the information I
presented, I can now address your questions.
1.
Based on the configuration of the UltracamFalconPrime
camera of (array size = 17,310 x 11,310 pixels, CCD
dimension = 6 um, lens focal length= 100 mm), a flying
altitude of 1,250 m (or 4,112 ft) results in an image GSD
of 7.5 cm (or 3 in) to support the horizontal accuracy for
a map with a scale of 1:600 (or 1 in = 50 ft). Here you
may notice that the flying altitude of 1,250 m (or 4,112 ft)
resulted from the use of a relatively long focal length of 100
mm may make it difficult to guarantee a class 1 vertical
accuracy of Root Mean Squares Error (RMSE) = 10 cm
expected for the safe production of 30 cm (or 12”) contours.
Unless you tested the vertical accuracy of products derived
from imagery acquired by such camera at that altitude, or
you were advised by the camera’s manufacturer otherwise,
I recommend lowering the flying altitude to around 915
m AMT (or 3,002 ft) if you are planning on compiling 30
cm (or 12 in) contours. By lowering the flying altitude to
915 m, the image GSD will be reduced to 5.5 cm. The new
flying altitude should guarantee the production of a map
with a scale of 1:400 (1 in = 30 ft) to 1:600 (or 1in = 50
ft) and 1 ft contours with accuracy that meets class 1 of
ASPRS standard horizontally and vertically.
2.
To produce a map with a scale of 1:1,200 (1 in =100 ft)
using softcopy compilation that meets class 1 horizontal
accuracy according to the ASPRS Map Accuracy Standard,
you can refer to Table 1 which suggests the use of an
image GSD of 15 cm (or 6 in). Here you need to fly the
project from an altitude of 2,500 m (or 8,224 ft) AMT.
Using such imagery, you should expect products with
horizontal accuracy of RMSE = 30 cm (or 1 ft.) according
to class 1 of ASPRS standard. Additional information on
relating image GSD and other map scales can be found in
previous articles of “Mapping Matters”. Table 2 provides
the geometrical characteristics of a few main large format
digital metric cameras available in the market today. It
can be used to compare the different flying heights used
by each camera to acquire imagery with GSD of 7.5 cm, 15
cm and 30 cm.
“Caution should be exercised when
it comes to some of the new digital
cameras that are equipped with lenses
with very long focal length”
“The latest generation of digital metric
mapping cameras brought added
quality to the mapping process with
enhanced optics quality, extended
radiometric resolution through a higher
dynamic range, finer CCD resolution,
rigid body construction and precise
electronics”
