Geog 303 - Intro GIS
Week 7 Lectures
Monday - prof was sick.....
Wed - went over last exam
Digital Terrain Models (DTMs)
- Introduction: Basically, we assume that objects such as the surface
of the earth are continuous, not discrete phenomena. As such, to fully
model the surface of the earth, we would require an infinite number of points.
Unfortunately, this is not feasible, so the question becomes: "how
do we represent the earth's surface?"
- Using a GIS, there are three basic methods of doing this: Digital
elevation models (DEMs), Triangulated Irregular Networks (TINs), and contour
lines.
- Contour lines are the simplest and most familiar to most people.
In a computer, they are typically a line with an attached elevation attribute.
In this form, they are of little use except as a static reference.
In short, you cannot do surface analyses on contour lines - they have to
be converted to one of the other digital terrain models (DTMs)
- as a quick side note, a DTM is a generic term which refers to any
computerised terrain model. DEMs and TINs refer to specific types
of terrain models.....
- Digital Elevation Models (DEMs)
- These are raster representations of the earth's surface. The
assumption is that there is a regular grid of spot heights which represent
the average elevation for each raster cell. As such, there is
a bit of averaging involved in a DEM. The amount depends on the
spatial resolution.
- Coverage can range from worldwide at very low resolution to less
than 1 metre resolutions for specific areas.
- DEM Creation:
- Conversion of contour lines. Very typical method, as elevation
information is often extracted from topographic maps. Simply
assign elevation information to the lines in a GIS and interpolate
to a raster grid. It should be noted that, for some GIS packages,
the lines need to be converted to a series of points with the appropriate
elevation tag.
- Airphotos
- Hard photogrammetric methods. Contour following or profiling.
Very labor intensive. Analyst can correct for terrain (plant/house)
inconsistencies.
- Soft Photogrammetric methods. Much faster. Must scan airphotos
first. However, the computer only looks at what's on top for
determining elevation - problems in areas of vegetation (treetops)
or buildings.
- Satellite Imagery - similar to soft photogrammetric methods. Radar
imagery or SPOT imagery
- Lasers (LIDAR)