Geography Reference
In-Depth Information
MrSID
JFIF (JPEG)
Esri BIL
Esri BIP
Esri BSQ
Windows Bitmap
GIF
ERDAS 7.5 LAN
ERDAS 7.5 GIS
ER Mapper
ERDAS Raw
Esri GRID Stack File
DTED Levels 1 and 2
ADRG PNG NTIF
NTIF
CIB
CADRG
TINs
A surface is a mathematical entity—in particular, it is a function of two variables. That is, if you consider
the Cartesian plane, with variables
x
and
y
, for every coordinate pair, there is one, and only one, third
value
z
. Visually, you can imagine the flat plane with a cloth billowed above it. The distance, measured
perpendicularly from the plane to the cloth is the z value. (Actually, some values of
z
might be negative,
which would indicate that part of the cloth was below the
z
= zero surface.)
An obvious example of such a surface is elevation
14
above sea level. Another is the daily pollution level
of a particular contaminant. A third might be wind speed at a given moment at a given altitude. For
every geographic point, there is a
z
value for these themes. Since there exists an infinite number of points
on a plane, there exists an infinite number of
z
values. Obviously, we could not store an infinite number
of values, even if we knew what they were, in a finite computer store. We, therefore, apply the usual GIS
techniques, which by this time you are used to, of storing some data and inferring information as we
need it. The triangulated irregular network (TIN) is such a device. TINs, which you met in Chapter 1, are
described in more detail in Chapter 9, which deals in part with 3-D GIS and is entitled the Third Spatial
14
Elevation is an obvious, but not perfect, illustration. A mathematical surface may have only a single value at a
given point and almost everywhere on Earth's surface this is the case. However, in a few places, like where there is an
overhanging cliff, the surface of the Earth can have three or more values. For GIS to take these into account would
create immense complication, so it gets ignored.
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