Measuring Distances on a Map

Measuring Distances on a Map

• Use a ruler to measure the distance between two places on a map
• Multiply map distance by scale to get ground distance in cm
• Divide by 100 to convert to metres
• Divide the distance in metres by 1 000 to convert to km
• Ground distance = Map distance x Scale

Co-ordinates to Locate Features

• Co-ordinates are intersections between lines of latitude and longitude
• They are given in degrees, minutes, and seconds
• There are 60 minutes in 1 degree and 60 seconds in 1 minute
• A network of lines of intersecting latitude and longitude is called a graticule

Map Index

• South Africa is divided into 1° x 1° blocks
• Each block is further divided into four blocks of 30’ x 30’
• These four blocks are labelled A, B, C, and D
• Each one of the four main blocks is further divided into four smaller blocks of 15’ x 15’
• These smaller blocks are also labelled A, B, C, and D

Locating a Feature or Place Using Co-ordinates

• Step 1: Identify the feature
• Step 2: Draw a line of latitude that passes through the feature
• Step 3: Draw a line of longitude that passes through the feature
• Step 4: Divide the 1’ space between two lines of latitude within which a feature or place falls into six equal parts
• Step 5: Estimate the seconds

Representation of Height on a Topographic Map

• There are four ways to indicate height on a topographic map: Contours, Trig Stations, Spot Heights, and Bench Marks
• Trig beacons are physical points on the land for which the exact height has been calculated and printed on a marker attached to the ground
• Benchmarks are similar to trig beacons and spot heights
• Spot heights show the exact height of a place and are shown with a dot marked on the map

Scale

• Scale is the number of times by which actual distances on the ground have been reduced to fit on to a map
• There are three ways of expressing map scales: Ratio Scale, Line Scale, and Verbal Scale

Contour Lines

• Contour lines join places that are at the same height above sea level
• Contour lines are printed as brown lines on topographic maps
• The contour interval is 20 m on 1:50 000 topographic maps
• Contour interval is the difference in height between two contour lines lying next to each other
• A darker brown contour line shows contour lines that are 100 m apart (index contour)
• Contour lines do not cross each other
• Contour lines give a clear picture of how the landscape of an area looks like

Steep and Gentle Slopes

• Contours on maps show many different landscape features
• Steep slope: Contours that are close together indicate a steep slope
• Gentle slope: Contours that are far apart indicate a gentle slope
• Concave slope: Contours that are close at the top of the hill and gentle at the bottom indicate a concave slope
• Convex slope: Contours that are gentle at the top of the hill and close together at the bottom indicate a convex slope

Contour Lines of Different Features

• Stepped slope: Contours that are close together at the top and bottom of a hill
• Cliff: Contours that are close together at the edge of a hill
• Waterfall: Contours that are close together at the edge of a hill and gentle at the bottom
• Ridge: Contours that are close together at the top of a hill
• Pass: Contours that are close together at the top and bottom of a hill
• Watershed: Contours that are close together at the top of a hill
• Gorge: Contours that are close together at the edge of a hill
• Mesa: Contours that are close together at the top and bottom of a hill
• Butte: Contours that are close together at the top and bottom of a hill
• Cuesta: Contours that are close together at the top and bottom of a hill

River Valleys and Spurs

• A river valley is the land a river flows over
• Spurs are narrow pieces of land that extend into or between river valleys
• Contour lines show the shape of a river valley and spur

Vertical Aerial Photographs

• An aerial photograph is a photograph of the landscape taken from above (aeroplane)
• Two types of aerial photos: Oblique and Vertical
• Oblique aerial photograph is taken by a camera angled to the vertical