Page 195 - Digital Analysis of Remotely Sensed Imagery

160 Cha pte r F i v e

5.3.4 Sources of Ground Control
There are two sources for obtaining the ground coordinates (E, N) of
GCPs, topographic maps and the global positioning system (GPS)
(see Sec. 5.6). Topographic maps were the exclusive source of ground
control prior to the advent of GPS technology. This source of ground
control is relatively cheap and efficient. Coordinates are interpolated
from analog topographic maps using a ruler within hours. Linear
interpolation is essential for those GCPs whose position does not
coincide with the kilometer grids on the analog topographic map. No
expensive equipment is needed, nor is a trip to the field necessary.
However, the accuracy of the obtained coordinates is subject to map
scale and the care taken during reading the coordinates. Coordinates
derived from topographic maps encompass two types of uncertain-
ties, uncertainty inherent in the map itself and uncertainty in reading
coordinates. According to the U.S. Geological Survey standards of
topographic mapping, horizontal accuracy must be within 2.5 m at
1:25,000 and 5 m at 1:50,000. For this reason a map of a larger scale is
always preferable to that of a smaller scale. Under ideal conditions the
minimum resolving distance of a naked human eye is 0.1 mm in inter-
polating the coordinates. This uncertainty is translated into a ground
distance of 5 m if the map has a scale of 1:50,000. In reality, of course,
the uncertainty in estimating the coordinates is much larger than this
theoretic minimum. This source of inaccuracy is irrelevant if the coor-
dinates are obtained from a digital topographic map. Determination
of coordinates from a digital map is usually much more accurate than
from an analog counterpart since no interpolation is required. Coordi-
nates can be read directly from the computer screen within minutes.
The accuracy of the read coordinates can be made higher through
zoom-in. However, the reliability of the obtained coordinates is subject
to the accuracy of the digital map. Any inaccuracy of the map propa-
gates into the acquired coordinates for both analog or digital maps
alike. The combination of source inaccuracy with reading inaccuracy
can easily create an uncertainty level around 10 m. Apart from the low
reliability of coordinates, topographic maps are limited in that they
are not readily accessible for certain parts of the world. Even if avail-
able, they may be obsolete or at an inappropriate scale.
An alternative method of obtaining reliable ground control is to
take advantage of GPS. The accuracy of GPS-derived coordinates is
governed primarily by the performance of the GPS receiver. With the
use of a competent GPS unit it is quite common to achieve an accu-
racy within 5 m under normal logging circumstances. This accuracy
level may be further improved through more processing. With proper
postprocessing, GPS-derived coordinates are much more accurate
than those acquired from topographic maps. Another added advan-
tage of using GPS is the ability to make use of distinctive ground
features that have been generalized on the topographic map because
of their perceived insignificance or small scale of the map. Thanks to