GEO 428: Visualizing Terrain
Tuesday, November 20, 2012
Cartography
Reduction of real world complexity
Generalization
Abstraction
On top of abstraction in data!
History
Without measures, difficult to represent
Until 19th century, little height measurement
Only about 60 mountains surveyed
Mountain symbols, nothing more
'Hillsigns'
History of Terrain Rendering
Dennis Wood, Power of Maps (1993)
Basic Tools
Visual Symbols
Point
Line
Area
Text
Graphic Elements
Hue
Color: red, blue, green...
Value
Lightness / Darkness
Texture: dotted, striped ...
Size
Symbol point, line width
Shape
Symbols
Spacing
In relation to other patterns / symbols / features
Orientation
Angle
Location
Position, often relative
Implications for Terrain
Points
Useful for terrain features (peaks)
Size, Shape, Value, Text
Lines
Frequently employed for continous surfaces
Elevation-oriented contour isolines
Aspect-oriented lines
Areas
Shaded fills
According to height or orientation
Or by physiographic form
Really a map of topographic classes
Lines
Contours
Oriented with elevation
Hachuring
Oriented with aspect
Contours
Lines of constant elevation
Parameters
Base Elevation
Interval
Line color, thickness, pattern
How many contour types on a USGS map?
Contouring
Essentially slice the landscape at each interval
Utility of Contours
Highly metric
Absolute elevations used
Reproducible method
Density varies according to relief
More detail in steep areas
Much can be gained from analysis
Though now digital raster data more useful
Example Contours
USGS Topo Quad
Depression Contours
Karst Landscape, Garrison Chapel Valley, Indiana
Hernando County, Florida
http://www.co.hernando.fl.us/plan/library/contours.htm
A Bad Contour Map
Without context, can be confusing
Which way does the water flow?
Choosing Contour Intervals
To illustrate local gradient
Be careful about using unequal intervals
Too wide
Not enough landscape detail
Too narrow
Not enough 'white space' for other thematic data
Very narrow implies unrealistic accuracy
Problematic if terrain variation differs across map
Interval Examples
Subset of Susanville, CA DEM
25 and 50 m. contours
Both 10 and 50 m. contours
Medora, 5m & 10m contours
Contouring Issues
Shows elevation, implies relief
Take some training to understand well
Contours rarely apparent on landscape
As a sampling technique:
Oversamples at contour intervals
Undersamples (none!) off intervals
If constant interval is used, fails at showing local relief
Tanaka Illuminated Contours
Differential shading of contours
Background is gray / white
Contours shaded with
sun angle
aspect
Results similar to Hillshading
Illuminated Contours
Tie to pre-digital modeling
Design
Art!
Homebrewed Tanaka
Does not modify width
See Kennelly & Kimmerling
http://www.mbmg.mtech.edu/gis/gis-hillshading.asp
Procedure
Develop Vector Contours
Convert Vector to Raster
High-cell resolution
Grow contour raster by 1 cell
Hillshade DEM
Hillshade x Raster Contours
Mess with Colors
Tanaka – Hillshade Comparison
Hachuring
Lines of constant aspect / slope
Most common 19th c. method
Do not show height, but steepness
Related to hillshading
Width, length may be varied
Hachuring Examples
Paterson's Road Maps
Hampshire (1785-1793)
Elements employed?
Afghanistan, 1838
Small Scale Map
http://www.lib.utexas.edu/maps/historical/bokhara_n_1838.jpg
Bad Ischl, 1911
Large Scale Map
http://www.lib.utexas.edu/maps/historical/ischl_1911.jpg
What shading technique is used?
Hachuring Issues
Hillshading easier to do
Does not portray elevation well
Does not portray slope magnitudes well
Portrays relative rates of change well
Easier for non-experts to understand?
Beware of hairy caterpillars!
Computer-Generated Hachures
Work by Kennelly & Kimmerling
Convert Slope, Aspect to Vector Points
Orient and shade (arrow) lines accordingly
Area Shading
Relief Mapping
Hypsometric Shading
Hillshading
Based on slope and aspect
Clear parallel to the lines case!
Relief Mapping
Variation of Choropleth map
Map divided into elevation classes
# classes determines precision
Each class colored differently
Very common approach
Esp. small-scale maps
Characteristics
Show absolute elevation, but not precisely
Only to the precision of the classification
Do not show local relief well
Class boundaries usually unimportant
But that is what map user sees!
Color Choice
Certain ramps very familiar
Green – Yellow – Brown - White
But no reason in principle for this scheme
May be confused with land cover
Hypsometric Shading Examples
Susanville, CA DEM Subset
Hillshading
Identifies reflectance from a light source
Terrain Angle – Sun Angle
Shadows
Light may not strike parts of surface
Illuminating 'Land Cover'
Representation in this NPS map
2007 Brochure
Terrain source: NED & SRTM
Land Cover Tricks
Landcover is largely NED-derived!
Shading reversed for high elevations
Distinguish from glaciers
Bathymmetry from NOAA contours
Scale & Representation Issues
Large Scale
Lines effective
Hachures especially effective
Small Scale
Much more problematic
Hachures ineffective
Hillshading ok
But gives little indication of relative height
Relief Mapping common
But with major issues
Mt. Shasta
http://www.siskiyous.edu/shasta/map/map/index.htm
GIS & Terrain Cartography
Don't try to do everything in the GIS!
Use Photoshop, The Gimp, etc.
Art, Science, or both?
National Park Service Tips
http://www.shadedrelief.com/
Summary
Graphic (Cartographic) Elements
Contouring
Hachuring
Relief Mapping
Hillshading
Representation Guidelines