Digital Elevation Model (DEM) - Completely GIS dan Remote Sensing tutorial - facegis.com
Digital Elevation Model (DEM)

What is a DEM ?

DEM: A digital representation of a topographic surface Local Example Global example

Also .. DTM: Digital Terrain Model; DSM: DIgital Surface Model Example

Topography: form or surface features of the land [versus Topology= form or connection of objects]


GIS and Remote Sensing tutorial of facegis.com         GIS and Remote Sensing tutorial of facegis.com

A DEM is an essential layer in the represention or analysis of any area with variable terrain: topographic map example

(For this reason, this topic appears in all UNBC Geomatics courses) e.g. Geog205 DEM lecture


1. Generation of DEM data

DEM data are somewhat different to other GIS layers as:

a. Topography is continuously varying (a surface)

b. It uses a third dimension (height)

c. It can be used to generate multiple components as well as elevation e.g. slope, aspect

Data generation for traditonal analogue maps:

 

Post 1945: Photogrammetry and stereoscopes-  overlap in air photos -> -> points or contours from 3D stereo model

Digital data generation: there have been two main methods:

    a. Digitizing of contour lines from maps
    b. Directly via digital stereo photogrammetry -> 'mass' points

    Also since 2000 these methods:

    a. Digital stereo satellite imagery Example - Svalbard
    b. LiDAR (Light Detection And Ranging) Example- Stonehenge

Most users NOW do not generate their own elevation data, they purchase or acquire files. These are acquired as:

a. Mass points (lattices)                    

b. Contour lines                 

c. GRIDs (interpolated from points or lines; or created from digital imagery)           

GIS and Remote Sensing tutorial of facegis.com   GIS and Remote Sensing tutorial of facegis.com   GIS and Remote Sensing tutorial of facegis.com

a. Mass points                                b. Contour lines                              c. GRIDs             

 

Table 10- 3: Available DEM files (BC)

Global 1:1,000,000 1000 ft GTOPO30  (grids)
Regional

1:250,000 / 1:50,000

100/25 m NTDB     (contours, grids)
Local 1:20,000 10-25 m BC TRIM (contours, points, grids)
Municipal 1:5,000 1 m e.g. City of PG (contours)

DEM 2000: Shuttle Radar Topography Mission (global to 60N, resolution 90m)


2. DEM Surface analysis

DEM surfaces for GIS analysis: must be GRID (raster) or TIN (vector)

Why?: points and lines are 'discrete', they must be interpolated to create a surface

 

Raster - Grids

Raster data are stored as elevation values per pixel:

   An Integer stores actual elevations in feet or meters (16 bit)     [may also be 32 bit real if decimals]]

    Advantages: simple data structure, fast, some analyses are computationally easy.
    Disadvantage: may create large files even in flat areas; features may be blurred by lost detail
Vector - TINs

Stored in Triangulated Irregular Network (TIN): a series of triangles .. heights and x, y at node (intersections)-: 'topology' is stored in the relationship between nodes, edges and slope facets.

The development of the TIN model is often attributed to Dr. Tom Poiker (SFU)

GIS and Remote Sensing tutorial of facegis.com    GIS and Remote Sensing tutorial of facegis.com

    Advantages:  variable data density depending on landscape,
    significant points or lines can be encoded e.g. peaks, ridges, valleys


    Disadvantages: more complex, needs more processing to generate,
    triangle facets are often evident in processing

 


3. DEM Derivatives

These products can be created from either a GRID or a TIN
a. Elevation:

Displayed as : Elevation (grayscale), color range (hypsometric tints) and contour maps (not analytical).

GIS and Remote Sensing tutorial of facegis.com

b. Shaded relief:

Describes the amount of reflected light from a surface assuming given light source; example

angle can be selected, but NW origin is traditional (315, 45)

- used in GIS map output, not analysis; see  lrdw.ca (imap)

 

c. Slope:

GIS and Remote Sensing tutorial of facegis.com

Calculated in degrees or  %  between adjacent pixels or from TIN.

Slope (degrees) = angle opposite rise/run   (arctan)
varies between 0 (flat) and 90 degrees (vertical);  ... a 1 to 1 slope is 45 degrees


Slope (percent) = rise/run * 100
varies from 0 (flat) to infinite (cliff);  ... a 1 to 1 slope is 100%

GIS and Remote Sensing tutorial of facegis.com

So for each pixel value, the slope (in % or degrees) is represented here by a shade of brown

i.e. steep slopes are shown as dark brown and low slopes as light brown

Note: GIS gives area, but can also give 'slope area' which is different from area for steeper slopes

 

d. Aspect:

The direction in which a slope is facing, measured by azimuth (0-360)

Calculated in degrees of azimuth from north in a clockwise direction, hence north is both 0 and 360.
Cardinal directions are 90 (E), 180 (S), 270 (W), 0 and 360 (N).

Flat slopes are given a unique integer for aspect (e.g. -1 or 9999);  WHY NOT 0 ??

Slope and Aspect image

 

e. 3D perspectives and fly-throughs:  e.g. Google-earth

The user specifies: example- earthdetails.com

  • viewer and target position.
  • azimuth angle of view and vertical angle.
  • vertical exaggeration and 'draped' layers.

terrainmap and virtual terrain project


4. DEM Applications

Cartographic output  / web mapping  e.g.  lrdw.ca                           

GIS theme analysis / queries of slope and aspect .. see labs

Recreation and landscape planning   Timberline                   

Line of sight / viewsheds

Steepest path - see labs


Volume estimation UNBC landfill example                                                      

Watersheds (using convexity / concavity):  BC watersheds atlas


Modeling      - what happens if :  mountain pine beetle example                                              

Visualization and Animation : visualization software

Flight Simulation / pilot training :  Google Earth 


5. Review

Things you should know after finishing this lecture:

  1. What do the initials 'DEM' stand for?
  2. What are the advantages & disadvantages of storing data in raster format?
  3. What are the advantages & disadvantages of storing data in vector format?
  4. Why does surface analyis require either a TIN or GRID (not contours)
  5. Name four direct DEM derivatives
  6. How are DEM data generated ?
  7. Where do users get DEM data from ?

Source: Source: http://www.gis.unbc.ca/courses/geog300/lectures/lect14/index.php