1. Statistical surfaces: DEM’s
Geog 4103, March 22

2. Real world phenomena represented as:
DISCRETE:
homogeneous or spatially averaged units, e.g. subwatersheds, counties, polygons
VECTOR
FIELDS:
discretized as grid cells or meshes
RASTER

3. What are surfaces?
Features that contain Z values distributed throughout area defines by (x,y) coordinate pairs
Z values can be any measurable phenomena that varies across space (temperature, elevation, precipitation, etc…)
called “field” like, or continuous data

4. What is a field? a conceptual model of geographic variation
at every point in the frame (x,y) there exists a single value of a variable Z
e.g. a field of temperature
e.g. a field of land surface elevation
the variable may be measured on any scale
temperature - degrees Celsius
elevation - meters above sea level

5. Field data are continuous
a field is spatially continuous by definition
values exist everywhere

6. Representation of field phenomena
A) CELLS
B) REGULARLY
SPACED POINTS
C) IRREGULARLY
D) CONTOURS
E) POLYGONS
F) TINs- Triangulated
Irregular Network

8. Isarithmic Mapping - used for continuous data Data measured at points
Derived data

9. Spatial sampling e.g. elevation Regular lattice restricted to
X,Y locations
Irregular lattice
not restricted
based on knowledge about how
smooth/rugged the surface is

10. Two methods of representing a surface inside a computer
Vector surfaces:
TIN’s (Triangulated Irregular Network)
Raster surfaces:
DEM (Digital Elevation Model)

11. RASTER vs. VECTOR DIGITAL ELEVATION MODEL

12. Triangulated Irregular Network (TIN)

13. Triangulated Irregular Network (TIN)
continuous mesh of triangles.
-triangles vary in size
based on roughness/complexity
of terrain.
- Large vs. small triangles

14. Raster Data Model
A raster representation is composed of a series of layers, each with a theme
Typically used to represent ‘field-like’ geographic phenomena

15. Raster Grid
but most common raster is composed of squares, called grid cells
grid cells are analogous to pixels in remote sensing images and computer graphics

16. Raster Resolution 1 2 4 3 = grid cell resolution
Spatial resolution = the distance that one side of a grid cell represents on the ground 1 2 4 3
= grid cell resolution
The higher the resolution (smaller the grid cell), the higher the precision, but the greater the cost in data storage

17. The DEM / DTM
Digital elevation models = a way of representing surfaces.
Quantitative model of a topographic surface in digital form.
data sets are continuous surfaces.

18. Elevation data Source of DEMs and TINs
Process of interpolation - creating continuous data from point data