DEM MEMORY ISSUES
MICRODEM requires the DEM be in memory (RAM) because:
- It speeds up the program considerably over disk access.
- It simplifies coding, since there need only be different
import routines and then all access of the DEM uses the
programs internal storage model.
- DEM storage is not standard, even at the level of storing by row or
- Many operations require a number of points
surrounding the desired point (e.g. reflectance, slope,
aspect), and multiple DEM formats would
complicate and slow down these operations.
- Operations like 3D oblique and perspective views, line of sight, and
viewsheds require sequential points along diagonal lines and not just rows
- The screen can display only a fraction of the points in a
DEM this size. A large DEM will effectively be subsampled to display on screen.
- Typical screen sizes are about 1000 x 1000 (now may 1920 by 1200
- Even my four monitor large
system is only about 4000x2000 pixels, athough I do have
dual 4K monitors.
- With subset, merge, and thin options supported in the
program, it should be possible to create a suitable DEM
of the desired region. Small scale DEMs should cover a
large area at lower resolution, while large scale DEMs
would cover small areas with better resolution. It is
probably appropriate to have two scales of DEM covering
the same region, a small scale DEM for regional synthesis
and large scale DEMs for particular areas of interest.
- You can have a small scale overview map, and use it to pick the large
scale DEM for specific areas of interest for detailed analysis.
Running out of memory can come from two sources. You should be
able to tell which source caused the problem by looking at the error message.
If it does not include words like "DEM", "column", or "Row", it is probably being
passed from the operating system to MICRODEM, and it is unlikely we can fix the
problem and you will have to be less "greedy" and "smarter" in what you want to
- You try to load a DEM, probably by merging, that is too big
for the current limits. You
should really think about whether you really need that level of
detail over such a large area (you cannot see it on screen, and
probably cannot also do meaningful analysis with it). This
is less than the amount of memory on your system.
- You try to create a map window on screen that exceeds the
limits of Windows or your graphics card. This limit has been
creeping up over time, and depends on your system's
characteristics, but overall I think it is plenty big (although
it will be much much less than the amount of memory you have).
You should have gotten a warning that this will take a lot of
memory, and noted that you cannot display it all on screen at
once and would have to use scrollbars. In this case, Subset
There is now a 64 bit version, and even larger DEMs
could be moved into memory.
Last revision 12/31/2012