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]

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)

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)

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

##### a. Elevation:

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

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:

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%

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.

## 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