Relief representation

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Relief representation
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Relief representation

• visible physical phenomenon
• new perspectives
• airline travel
• airborne and satellite platforms
• ocean and extra-terrestrial mapping - same
  methods applied

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Special problems

• altitude variation from the spheroid is small
  relative to horizontal distances
• Everest (8,848m) on a 3D model _at_ 110,000,000 lt
  1mm high
• human perspective dominates interpretation

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Special problems (cont.)

• large and small scale maps problematic
• thematic maps also
• potential loss of clarity and detail
• - best solution could be expensive
• - dominates map
• - source of criticism

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Development

• search for balance
• mathematical representation v portrait of
  landscape
• portrayal of accurate values - least effective
  visually
• perspective views associated with landscape
  painting (C15th to C18th)

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1.1 Hachuring

• - light source assumed to be vertical in most
  cases
• - lines along direction of slope
• - width and/or spacing relate to steepness
• - formalised by Johann Lehman 1799
• - 19th century topo. maps
• - vary in shape

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1.2 Advantages

• - limited relief data required
• - detailed slope change shown
• - suited to engraving
• - convenient for field sketches
• - supplement to contouring

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1.3 Disadvantages

• - manual skill dependent
• - series maps lack uniformity
• - detail obscured
• - difficult to automate
• - no absolute height variation provided

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Hachuring closed polygons
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Hachuring all contours
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2. Hill shading

• slope shaded in proportion to intensity of slope
• assumes artificial or imaginary light source
• development of air brush and printing important
  during 19th Century
• smooth tonal variation possible

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2.1 Assumptions

• - light from constant direction and elevation
• - light reflected from a surface with constant
  reflective properties
• - map is viewed orthogonally
• - light source (elevation/direction)
• - usually NW

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a) model photography

• - illuminate relief model
• - photograph
• - light parallel with landforms
• - excessive/insufficient shadows

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b) manual hill shading

• - fit visual appreciation of photography
• - interpret air photography
• - superimpose stereo model on base map
• - shade direct on to outline
• - inconsistent
• - requires skill
• - creates portrait of landscape
• - easily visualised (if done well)

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Mount St. HelensShaded relief map45 degree
elevation
NW Light source
5120000.00
5118000.00
5116000.00
5114000.00
5112000.00
5110000.00
558000.00
560000.00
562000.00
564000.00
566000.00
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Mount St. Helens shaded relief map90 degree
elevation
Vertical light source
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NW Light source

• Mount St. HelensShaded relief map10 degree
  elevation

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Mount St. HelensShaded relief map45 degree
elevation
SE Light source
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Automatic shading methods

• Lambertion reflection
• assumes ideal surface
• reflects all light
• Lommel-Seeliger
• based on light reflection from surface
• between ideal diffuser and real surface

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Mount St. HelensShaded relief map Lommel-Seelige
r intermediate method
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Mount St. HelensShaded relief map Lambertian
Reflection
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Gradient smoothing

• Midpoint difference
• computes gradient at the centre of each grid cell
• less smoothing
• Central difference
• averages slope and orientation across three
  adjacent grid nodes
• blanks edge cells

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Z - scale factor

• increasing scale factor enhances shading effect
• useful for low lying relief
• default usually 1/10th diagonal distance

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Mount St. HelensShaded relief map 2 x Z
scale factor
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Mount St. HelensShaded relief map Colour scale
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3. Contouring

• - assume land continuous
• - assume simplest gradient
• - bound to be erroneous
• - most metrical method
• - suited to measurement
• - does not dominate map
• - less reliant on manual skill

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3.1 Index contours

• - every 4th or 5th line - thicker
• - labelled
• - visual reinforcement

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Index contours
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3.2 Standard or intermediate

• - at basic contour interval
• - finer line (half)
• - not labelled

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3.3 Supplementary

• - when required
• - pecked/dashed

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3.4 Interval

• - equal interval
• - depends on accuracy of data

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3.5 Labelling

• - top of figures up-slope
• - in a line
• - label isolated contours

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Contour labelling
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3.6 Disadvantages

• - high density data required
• - difficult to visualise
• - requires map user to be experienced

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4 Layer tinting (hypsometric tints)

• - coloured zones of equal elevation
• - dominates map
• - simplifies elevation

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Layer tinting - grey scale
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4.1 Problems

• - evenly balanced/perceptively different
• - all map elements affected
• - colours for lowland must not affect detail
• - avoid abrupt changes in colour (lose continuous
  surface)
• - may not match physiographic regions
• Colour association

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4.2 Advantages

• - easily visualised
• - attractive
• - colour association can be used

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Hypsometric tints
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Criteria for selection

• i) Density of information
• - height data other detail
• ii) Map use
• - visualisation important?
• - measurement required
• iii) Map user - map user experienced?
• iv) Cost

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Surface modelling