Check out this link for more information:
http://www.nrcan.gc.ca/earth-sciences/geomatics/geodetic-reference-systems/9052#geoid_models
2. Depicting elevation.
- earliest method: pictures of hills (since 2000 B.C.)
- Hachuring: slopes shown by many small lines pointing downhill. Steeper slope, thicker hachures, so steep slopes look darker. Common 1700-1900.
- Contours: A contour is a line joining points with the same elevation. Popular since 1800.
- Shaded relief: shows hills and valleys as if illuminated by sun.
Check out this link (PDF file) which includes a section about contour maps.
And... this link about interpreting contour maps. Note: not all contour maps are of elevations!
1. Measuring slopes on a contour map:
- Find the point at which you want to know the slope.
- Find the elevations of the contour lines on each side of it.
- Measure the distance between the contour lines (at right angles to the lines themselves).
- Slope = "rise over run", or vertical height increase over horizontal distance.
- Example: if two contour lines are 300 m apart (use map scale to find the distance between them!) and they represent elevations 50 m apart, then rise = 50 m, run = 300 m. Slope = 50/300. But we don't write it that way...
- (1) Percentage slope: do the division, multiply by 100: (50/300) x 100 = 17%
- (what does this mean? - for every 100 m you move horizontally, you climb 17 m higher.)
- (2) slope as an angle: do the same division: 50/300 = 0.16667. This is the tangent of the angle of slope.
- We need the angle whose tangent is that number. On a calculator, having found that number, press INV TAN or 2ND TAN (or however your calculator gets to the "inverse tangent" function).
- In this case the angle is 9.5 degrees.
- NOTE: The 'run' can be the distance between any two points, not just between two contour lines. You just need to know the elevation of each point and the distance between them.
Check out this link about calculating slopes.
2. Collection of data:
- Values measured only at specific locations.
- Impractical to measure everywhere.
- Best results if we measure at significant locations:
- example:
--- estimate elevation of a point on a hillside.
------ easy if we know the elevation at top and bottom of the hill.
------ impossible if we have elevations only on hilltops or only in valleys.
--- surveyors measure significant elevations:
------ points at bases of hills,
------ along tops of ridges,
------ anywhere the slope of the ground changes.
--- assumption: between those places the slope is constant
- Easy to measure significant points if we know where they are.
- If we cannot see the area, measured points may not accurately reflect the shape of the surface.
--- example 1: soundings of sea or lake bottom, soil pits, drill holes.
--- example 2: 'invisible' data sets like population density, temperature, rainfall, pollution concentration.
3. Sampling
- measuring at intervals over an area
- three methods:
--- Random : points at random locations
------ coordinates read from random number tables or computer random number generator.
------ measurements made at the locations given by those coordinates.
--- Systematic : points on a regular grid
------ located by using surveying equipment
--- Significant Points : points known in advance to be important
------ identified from air photos, inspection, theory.
4. Begin contouring
STEP 1: Select contour interval.
- must be sufficient to show level of detail required
- limit if needed to reduce clutter or drawing time
STEP 2: Visualize pattern of hills and valleys.
- remember water bodies are level so coastlines are parallel to contours.
- the sea coast IS the zero contour.
STEP 3: Connect each point to its nearest neighbours.
- with imaginary lines (or real if it helps)
- DO NOT cross obvious valleys or ridges.
STEP 4: Choose one contour level to work with.
- (a) find a pair of points which straddle that contour level.
--- estimate the position between them where the ground would be at the level you want.
--- example: contour level 400 m, points at 380 m, 460 m.
------ rough estimate: 400 m is probably closer to 380 m point.
------ betterestimate: 400 is 20 m above 380
------ 1/4 of the elevation difference from 380 to 460.
------ so 400 m contour is 1/4 of the way between the points.
- (b) repeat (a) for all other points at that contour elevation.
- (c) connect all those points with one or more smooth curves.
- (d) if the point elevations are far apart there may be several contours along the line. Space them equally.
STEP 5: Repeat Step 4 for all other contour levels on the map.
REMEMBER:
- Rivers cannot flow uphill. Double check this on your map.
- Rivers cannot cross a contour line twice.
- Contours often have a V-shaped kink pointing upstream where they cross a stream.