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# Full text

Title
Mapping without the sun
Author
Zhang, Jixian

Satellite position
a tree
H=V/tan(EL) (1)
where H is the horizontal displacement
V is the vertical displacement
EL is the sensor elevation angle
Here suppose the EL at imaging time is 75 degrees, then a
mountain peak with 1,000 meters will has a horizontally
displacement of 267.949 meters, this displacement is important
and inevitable.
As we know, the changes of satellites’ parameters are capsule,
the horizontal displacements generated by this also very small
even with a large height. For instance, suppose the sensor
elevation angle changes form 75 degrees by arc-second (one
arc-second equal to 1/3600 of a degree) to 75.00028 degrees,
then H changes from 267.949 meters to 267.944 meters, an
insignificant change. It illustrates that vertical displacement has
no improvement for satellite pointing calibration, so a flat area
is more likely to be selected as a test range for it is as good as
mountain. A flat area also more convenient to arrange GCP
(ground control points). Good GCPs are related to the result of
calibration also. To select culture features as GCPs (Gene Dial,
Jacek Grodecki, 2003) is an effective way for build good GCPs
which can easy be found on the image, be permanent, and be
low-cost.
2.3 Area condition
This condition depends on satellites’ swath width. The overall
size of a test range ought to suitable for satellite sensor so that
geometric calibration can be done on a full scene at once.
Choosing test ranges based on the swath of a satellite is one
thing to be thought.
2.4 Cost condition
Cost condition is a subjective factor; to gain most profit with
least input is one principle for doing everything. So if there are
many areas satisfying the three conditions list above, the one
with the lowest cost must be chosen. Several facts to be
considered in this, such as traffic, GCPs, convenience.
3. PROCEDURES
Those four conditions are kept as rules when selecting a test
range in China and the four analyze stages usually are carried
out from weather analysis to terrain analysis, then consider the
area condition, last take cost into account.
3.1 Weather analysis
Cloud is the emphasis in this stage. Analyzing this side needs
meteorological datum. We obtained relative data from China
Meteorological Administration. It is statistical data from year
1951 to 2000. Cloud data is a picture about year average cloud
amount distribution of the whole country. It tells us a rough
result that where has the high probability of cloud- free skies.
But no more detail geographic information in the picture. For
obtain detailed geographic information, a superposable analysis
with administrative divide map is necessary.
First, using the ERDAS software to let the cloud amount
distribution map has geographic information. Later in Arclnfo
environment, it easy to do registration, and then select cloud-
free zones with accurate geographic information. Plenty of
zones satisfied this condition, choose some of them and then do
terrain analysis on this result.
3.2 Terrain analysis
The purpose of this procedure is to judge landscape of zones
which be selected in the previous stage and screen out flat areas