Terrain Data Models

Question 1

What are the 2 types of digital elevation models?

Answer 1

◦ Raster Digital Elevation Model (DEM)

◦ Triangular Irregular Network (TIN)
◦ (ArcGIS Terrain is a multi-resolution TIN

Question 2

What is Raster DEM? Advantages and disadvantages

Answer 2

Represent surface with an elevation matrix

Cell size (resolution) determines the level of detail

Usually interpolate grid elevations from irregularly spaced data point
Once you have a regular grid, you can use it to model 2D and 3D graphic display, and for surface analysis

Question 3

What are TIN models?

Answer 3

triangulated irregular network

Link neighbouring data points to
form triangular lattice

For each triangle store:
◦ vertex identifiers
◦ adjacent triangle identifiers

No interpolation required since
using known data points directly in
model
◦ analysis based on known points rather
than interpolated grid elevations
(ESRI 2018)

Question 4

what are the Advantages and Disadvantages of TIN model?

Answer 4

Advantages:
◦ no interpolation
◦ no filtering of data points
◦ adapts naturally to terrain roughness
◦ no redundant data if input points carefully selected

Disadvantages:
◦ more complex data structure
◦ must explicitly define topology
◦ edge-triangle, edge-node
◦ spatial searching possible but more complex than with regular grid
◦ operations such as smoothing more complex since must fit curved surface to
arbitrary set of triangles
◦ Cannot represent vertical surfaces

Question 5

what are the advantages of disadvantages of Rater DEM models?

Answer 5

Advantages:
◦ Simple data structure
◦ Spatial relationships (topology) implicitly defined by position of data values
in the grid
◦ Easy to implement neighbourhood functions based on rectangular window
or kernel

Disadvantages:
◦ Data volume large if you use a fine resolution
◦ Not adapted to terrain roughness
◦ There may be redundant data
◦ Filtering and interpolation is often required to generate the regular grid
◦ Grid values used for analysis subject to interpolation errors

Question 6

what are the strengths and limitations of TIN?

Answer 6

Strengths
◦ Methodology easily updated as scenarios improve.
◦ Applied in other coastal areas to assess potential impacts on a range of land uses and
shoreline development issues.
◦ Could provide general assessments of short term and seasonal shoreline change.

Limitations
◦ Data availability, accessibility, and cost.
◦ Static model incapable of reflecting dynamic shoreline processes.
◦ Software and data limitations restrict water level scenario representation to 10 cm intervals.

Question 7

Applications of TIN vs Rater DEM

Answer 7

Raster DEM:
surface morphology
◦ slope, aspect, curvature
◦ slope maps
◦ aspect maps
◦ hill shaded maps

interpolation
◦ contour maps
visibility analysis
◦ line of sight
◦ viewsheds

cut and fill
◦ difference between DEMs
◦ volumetric calculations

hazards research
◦ slope stability
◦ flood potential

remote sensing
◦ interpretation of land cover
◦ radiometric modeling

facility location
◦ constraints on many types of development due to
  ◦ flood potential
  ◦ slope
  ◦ groundwater recharge
  ◦ aspect
  ◦ etc.

Applications of TIN:
Changes In Climate And Water Levels: Using GIS to model Lake
Huron shoreline change at Goderich, Ontario

Question 8

Raster DEN vs. TIN

Answer 8

You know that TIN is a vector-based representation whereas DEM is represented as a raster from grid of squares. Actually TIN is a type of DEM and derived from the raster DEM.

The TIN representation has information about altitude, slope and aspect and you can use them to extract the areas you require.

Raster DEM

• Data redundancy in areas of uniform terrain
• Inability to adapt to areas of differing relief complexity
• Exaggerated emphasis along the axis of the grid

TIN

• Non-redundant data
• Allows extra data in complex areas and less data in non-complex areas
• The ability to use natural features as break-lines
• Of course, you have to like triangles

Question 9

Data inputs for raster dem

Answer 9

Feature Types:
Points
◦ spot heights

Lines
◦ contours
◦ profiles
◦ 2d lines

Areas (polygons)
◦ window or boundaries
◦ “holes” e.g. lakes

Question 10

Data inputs for TIN

Answer 10

Mass Points
◦ Point height measurements that become nodes in the network.
◦ The primary input into a TIN
◦ Determine the overall shape of the surface.
◦ Can be extracted from point, line or area features (i.e. the points in a contour
line)

Breaklines
◦ Lines with or without height measurements.
◦ Become sequences of one or more triangle edges.
◦ Typically represent either natural features such as ridgelines or streams or
built features such as roadways.
◦ Hard or Soft

Polygon surface feature types
◦ Clip polygons - Define a boundary for TIN creation or analysis. Data that falls
outside of the clip polygon is excluded from the TIN creation and analysis
operations.
◦ Erase polygons – Analogous to an interior clip polygon. Input data that falls
within the erase polygon is excluded from the interpolation and analysis
operations (like a donut).
◦ Replace polygons – Value for the boundary of the polygon is used for all
interior elevations. Could be used to model a lake or a other human-made
level surfaces.
◦ Fill polygons – Designates an area to be filled by assigning an integer attribute
value to all triangles that fall within the fill polygon. The original surface
heights of the triangles are unaffected.