Department of Urban Studies and Planning Spring 2002 Department of Architecture Site and Urban Systems Planning 11.304J / 4.255J

Exercise 3

CONTOURS, SLOPE AND ASPECT MAPPING -

Task 1. Complete the contours on Map A from the spots elevation provided.

Draw an East-West section (on a separate sheet) and indicate the section location on the map.

Task 2. Construct and color a slope map for Map B using the following slope categories:

0 - 10% - light yellow 11 - 20% -dark yellow 21 - 40% -light brown >41% -dark brown

Calculate the approximate acreage in each category.

Draw a map legend which indicates the slope symbols and categories.

Task 3. Construct and color an aspect map for Map C using the following aspect categories:

NE - light blue NW - light violet SW - light yellow SE - light green

Draw a map legend which indicate the aspect symbols and categories.

Format: • All work to be reviewed must be neat and well organized. • Each mapping exercise should be performed on the map provided. As

corrections will occur, it is suggested that you make extra copies of each map to be used as a working medium.

• The maps are to be handed in stapled together with a cover page indicating your name, course number and year.

Due Date: .

NOTES:

1. Construction of a contour map from elevation data:

Given: contours are lines of equal elevation (known points).

Problems of interpolation between known points a. Estimate approximate change in elevation by measurement perpendicular to

contour lines (steepest slope of landform). b. Difficult to estimate rate of change in slope - stream gradients tend to be more

consistent in rate of change. Because contour lines are points of known value, we generally interpolate between these lines assuming a certain degree of uniformity.

c. Approximate segments of unmapped contour lines (elevations) near known points.

d. Landforms have a characteristic appearance influenced by the underlying rock and drainage system - changes in appearance generally occur with changes in rock type and drainage regime.

e. It is generally easier to construct (interpolate) contours in areas where there is more information per unit area, i.e.,, steeper slopes. f. It is generally easier to start in known areas and move to unknown areas.

2. Construction of a slope map

Slope inclination is a measure of the relationship between change in horizontal and vertical distances between at least two points usually expressed in percentage of slope.

percent slope = change in elevation (Rise) distance (Run)

USGS topographic contour maps (with contour interval of 20) do not provide information in a form suitable for detailed planning and design purposes.

Slope is important in identifying constraints and evaluating potential environmental impacts related to landform alteration. Major constraints can be tied to grades/inclinations that are either too steep (to reasonably construct structures, roads, etc.) or too gentle (for playfields, etc.). Major impacts related to inclination include erosion/loss of soil/non-point source pollution and slope failure (linked to weak rock, soils with low bearing capacity, steep slopes, etc.). Coupled with aerial photographs, slope maps are excellent tools to look for potential erosion areas, drainage patterns, landform and soil patterns, land use suitability, etc.

Slope maps are isoline (lines of equal value) maps of selected slope categories.

To construct a slope map we must select specific slope categories to map and know the scale of the map and the contour interval.

The easiest approach to mapping slope is to measure slope between two contour lines - such measurements are always perpendicular to the axis (direction) of the two contour lines.

The easiest method of mapping slope is with a "tick sheet", i.e., a scaled reference sheet of selected slope isolines drawn on a card.

Formula for measuring distance between contour lines to determine slope isolines:

D = 100(CI) D = distance in inches between contour lines (P)(S) CI = contour interval in feet

P = % slope [isoline] S = scale of map (in feet equivalent), e.g., 1" = 400'

e.g. for 10% slope on 1” =400’ with CI = 20 ft.

D= 100(20) = 0.5" between contour lines (10) (400)

Hints for Mapping Slope

• Slope measurements between contour lines- safe approach. • Slope measurements across several contour lines - more difficult; requires interpolation. • At crests and knolls - assume a constant or lessening slope (but be conservative). • At footslopes/bottomlands - assume a constant or lessening slope (be conservative). • At streams - assume a constant stream gradient

3. Construction of an aspect map

Aspect - equal to the orientation of a landform slope relative to the cardinal points, e.g., southern aspect, northern aspect, etc....

Aspect maps are isoline (lines of equal value) maps of selected aspect categories, e.g., southwest, southeast, northeast, northwest, etc.

Categories should be selected as a function of climatic concerns; remember, there is an interaction between aspect, slope and relief that will change microclimatic conditions (shadow patterns and frost pockets).

Aspect isoline maps are constructed on the basis of contour line data, not slope data. Aspect change is measured relative to the change in direction (perpendicular) of contour lines.

Hints for Mapping Aspect

• Orient map so that parallels and perpendiculars may be used to mark points of tangency to contour line direction; think of water flow across the land - using arrows may help.

• Follow landforms from top to bottom or bottom to top along point of tangency • Aspects are easier to draw in steeper slope situations because of an increase in information.

e.g., for 4 QUADRANT ASPECT MAPPING [NW, NE, SW, SE]

set up map so that north is aligned to top of board;

east/west aspect isoline is then parallel to top of board;

north/south aspect isoline is then perpendicular to top of board.

* use minimum mapping units, i.e., minimum widths & sizes (for example: do not map areas smaller than one acre).

Exercise 2