- Mapping Terrain
- Terrain (topography, relief, elevation, bathymetry) - 'shape of the ground'
- Vertical Datum
- vertical datum = reference surface for elevations
- MSL (=geoid) ... various models exist
- Methods for Depicting Terrain
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- fixed intervals (5, 10, 20, 50, etc. - depends on scale and terrain)
- types of contour lines
- intermediate - "regular contours", lighter lines, not labeled
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- Profiles
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- align strip & mark contours (label as needed)
- mark start and end points on map (i.e. A & B)
- align edge of paper along the line ( A -> B)
- make a tic mark where contours cross paper
- label contours as necessary
- mark & label special features (hilltops, creeks, roads, etc.)
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- transfer tics from strip --> graph paper
- label the Y axis with elevations (exaggerated?)
- label X axis with start and end points
- use "tic marked paper" to transfer elevations to the profile
- join tics with a smooth line
- transfer and label any special features
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- Calculations
- elevation of a feature
- eyeball it for a rough guestimate
- interpolate btwn the 2 contour values ...
- 2 ratios ... "dash = dash" ... or proportion
- Slope
- example
- contour interval is 20 metres
- line on map crosses 4 contour lines (= 80m elevation)
- line is 420m (horizontal)
- slope = 80m / 420m = 0.19 (or 19%)
- slope in degrees = arctan (0.19) = 11 degrees
- Slope Distance
- trig
- remember
- COS = adj. / hyp. = HD / SD ... HD as a % of SD
- rearrange formula ...
- SD = HD / COS (slope degrees)
- SD = 420m / COS(11 degrees) = 420m / 0.982 = 428m
- Pythagoras
- a^2 + b^2 = c^2
- elev^2 + HD^2 = SD^2
- 80^2 + 420^2 = c^2 ... = 428m
- Spot Ht --> Contour
- interpolating ... a proportion (again!!)
- example
- spot hts: 128m & 101m
- where is 120m contour line?
- "how much more" / "total change"
- how much more = 120 - 101
- total change = 128 - 101
- 19 / 27 = 70%
- ... it's 70% the distance from 101 to 128
- Grade Line
- idea - sketch a proposed road (trail) at a fixed gradient (steepness)
- 1) determine "target spacing" btwn contours
- e.g. 15% max, 20m contours, map 1:5,000
- a) calc. HD (m) for your contour map that = x%
- 2 ratios ... "dash = dash"
- convert slope to rise/run ... 15m/100m
- equate to contour interval ... 15m/100m = 20m/??m
- ?? = 133m (or 13,300cm)
- b) convert to map dist ...
- map / RW = scale
- x cm / 13,300cm = 1/ 5,000
- x = 2.7cm
- this is the "target contour spacing" on the map
- 2) sketch the route
- locate points btwn contour lines (i.e. 2.7cm apart)
- "connect the dots"
