454
dotted with lakes. If the 4.3% of the scene with 1,426
computer-identified water bodies is representative of the
Arctic Coastal Plain area the system might identify
as many as 28,000 water bodies on the terrestrial
estate contained within this 1 scene.
Lake Calculations (Subroutine LCALC). Each lake com
pletely contained within the scene area processed has had
its centroid (center), area, perimeter, and crenulation
calculated. The PIXGEO subroutine is called to convert the
Landsat polar coordinates for the lake centroid to latitude
and longitude through map projection algorithms. PIXGEO was
developed by a number of people cooperating within the USGS
Cartography Division of Reston, Virginia, and EROS Data
Center Digital Image Processing Group at Sioux Falls, South
Dakota. Preselected ground control points are used in NASA
Goddard Space Flight Center and EROS Data Center software
systems to produce geometrically corrected Landsat scenes.
These corrected data make this lake system unique and par
ticularly useful. A user will be able to obtain an EDIPS
corrected computer compatible tape that, when fully proc
essed, should be capable of lake basin centroid calcula
tions that have latitude and longitude accuracies to
+ 60 m (McCormach and Elliott 1980). A new subroutine
being developed for geometric correction of Landsat IV
scenes should provide more accurate positions with greater
computer efficiency. These systems can provide a lake
data file suitable for geographic retrieval of information
at accuracies near those of inch to the mile or smaller
scale map bases. The original centroid calculation is
listed in decimal degrees for latitude and longitude.
Decimal degrees are converted to degrees and decimal
minutes for lake data retrieval, but the decimal degrees
to 5 places for both latitude and longitude provide the
unique identity for each lake.
Area is the second most useful lake parameter calculated.
Area is calculated in km 2 , accurate to 5 decimal places, to
accommodate the resolution of a .00325 km pixel. Lake
area is an easy, inexpensive, and useful parameter to cal
culate .
Perimeter is calculated through the rigorous and time-
consuming process of following the perimeter samples around
the computer generated lake shore. Removing this calcula
tion from the program might save a significant amount of
computer time, but this has not been tested.
Crenulation is calculated from area and perimeter values to
provide an index of shoreline development. A value of
1.000 is the idealized smooth shoreline of a perfectly
circular lake. The index value increases from 1.000 as the
shoreline perimeter/crenulation increases.
Additional lake surface characteristics, such as length and
azimuths of major and minor axes, might be added to computer
calculations in the future. At this time, storage space and
computer cost have limited this system to centroid, area,
perimeter, and crenulation. Perimeter and crenulation are
not particularly important lake criteria but show the