Planetary geomorphology methods - Introduction to Planetary Geomorphology - page 30

2.9 Electronic data records (EDRs)

EDRs are the files from the mission (this refers to all data,
not just images). For images, there are different levels of
processing, as outlined below.
Level 0 refers to the raw data (no processing) as
received from the spacecraft. This version is generally
preferred by scientists who need to conduct quantitative
studies, such as photometry (precise measurements of
surface brightness), for which they have their own algo-
rithms for customized processing. Level 0 is often the
preferred archival method to preserve the original files in
order that algorithms developed later can be applied.
Level 1 data are decompressed. To make data acquis-
ition and transmission more ef cient, various data com-
pression techniques are applied; these can be either
hardware techniques built into the imaging system or
software techniques that can be updated on board the
spacecraft. For example, all pixels that have a DN of
zero (black) might be automatically eliminated from the
data transmission. Compression is either lossless (preserv-
ing all the data) or lossy, in which some data are lost. In
level 1, the compression is “ reversed ” to restore the orig-
inal image to the extent that this is possible.
Level 2 data have the calibration files applied to pro-
duce radiometrically corrected images in which the
brightness levels are correctly given, taking into account
illumination, etc. On some missions, radiometric calibra-
tion is done on level 1 files.
Level 3 data have been custom processed for speci c
uses; for example, images might now be in some uniform
map projection.
Level 4 data are further processed and might be merged
with other data; for example, level 4 images might have
topographic information incorporated.

Figure 2.20. Lunar Orbiter photographs were transmitted and
reconstructed by assembling strips of film, resulting in the distinctive
pattern seen in (a); mosaicing and image processing results in a
smooth, continuous image as shown in (b).

2.10 Cartography

Maps are essential for exploration. No doubt, early
humans scratched simple maps in the dirt to describe
hunting grounds, clan boundaries, and other geographic
locations critical for their survival. Planetary maps are
generated from image mosaics using conventional carto-
graphic projections. The USGS is supported by NASA to
produce maps with a variety of scales, projections, and
portrayal methods. Most of the terrestrial planets and the
satellites that have been adequately imaged have been

Pixel scale can also be erroneously set with the image
processing tools commonly available on computers. Some
of these tools resize the image, in effect adding pixels to t
some format, and leading to an imaginary pixel scale that
is smaller than the actual pixel scale of the original data.
When using such tools, it is best to use a “ sanity check, ”
keeping in mind that the pixel scale cannot be better than
that of the original image.