package originally written at the SUNY Computer Science 
Department (Frieder) and modified for DEMGS by the GIS Lab 
oratory. This virtual emulator package is a paging mechan 
ism that brings into the main core 64 * 256 word pages when 
DEMGS is initialized. E ery package of DEMGS keeps track 
of the core resident bands so as to keep the processing as 
efficient as possible. Therefore, in this sense, when the 
system is first entered, about 33 band/quads are brought 
into memory. The system keeps track of the location of 
every band/quad in the page layout such that when a band 
that is not in memory is requested (which occurs rarely), 
the system 'knows' exactly where to locate it in the data 
base. The data compaction is very much system dependent but 
very efficient. After the DEM source file is entered into 
DEMGS, this file is divided into two sub-files. One is a 
special purpose header file (II-file) which is very seldom 
used, and the other is an elevation or Z-value file which is 
a disk resident data structure that is handled by the vir 
tual memory emulator. 
A package called GENBLK (GEnerate BLocKs) compresses a reg 
ular 7.5 minute USGS quadrangle of about 1,155,870 bytes to 
an H-file of about 54,200 bytes and a Z-file of about 
500,224 bytes. This 7.5 minute quadrangle covers approxi 
mately 150 square kilometers. The H-files and the Z-files 
of all the source files are kept, and the source file is 
purged from the data base. If the source file is ever need 
ed, DEMGS can merge and re-code the II-file and the S-file 
to obtain it. These compacted files are accessed by all 
the DEMGS packages. 
Besides data compaction and data structuring, GENBLK also 
generates some additional parameters necessary for the con 
version of the data to screen coordinates, a tedious and 
time consuming process done only once for all DEMGS files. 
The average time of generation of the H-file and the Z-file 
is about seven minutes, while the average processing and 
display time of any other package is about three minutes for 
a 360 by 460 grid. 
ISOPLETII AND CHOROPLETII MAPS 
DEMs' can provide the same basic elevation information found 
in standard topographic sheets. There are two principle 
mapping methods which have beed used to display this infor 
mation: the choropleth, and isopleth maps. DEMGS incorpor 
ates both mapping methods and accomplishes these displays 
with an emphasis on both speed and a minimum internal 
memory requirement. 
The grid size of the DEM and the resolution of the display 
device are approximately the same. This allows the various 
algorithms to utilize a one cell equals one pixel approach 
and thus eliminates the need for interpolation between 
grid points. The memory requirements arc also minimized in 
that no more than two bands of data have to be in core at 
one time, although up to 30 bands are actually core resident 
(the extra bands not 'directly' used are available for 
future expansion).