Chapter 1. Topographic Maps: Introduction and Conventional Signs FREE
1.1 What is a topographic map?
A topographic map is a detailed, accurate map of a small part of the land surface. It shows both natural features (rivers, hills, vegetation) and constructed features (roads, railways, buildings, dams) using symbols, colours and contour lines. Unlike an atlas map of a whole country, a topographic map zooms in close enough for you to find a single farm dam or a school. In Grade 9 you work mainly with the South African 1:50 000 topographic map series. The scale 1:50 000 means that 1 cm on the map represents 50 000 cm (= 500 m) on the ground, so 2 cm on the map is exactly 1 km in the real world. You will practise scale calculations in Chapter 2.
1.2 The South African map series and map codes
The whole of South Africa is covered by a national series of 1:50 000 maps, produced by the Chief Directorate: National Geo-spatial Information (NGI). Each sheet has a map code that tells you exactly where in the country the sheet lies.
Worked example: decoding the map code 3318CD.
- 33 = the block of land starts at latitude 33 degrees south.
- 18 = the block starts at longitude 18 degrees east.
- That one-degree block is divided into four quarters labelled A, B, C and D. The first letter, C, selects the bottom-left quarter.
- That quarter is divided into four again, and the second letter, D, selects the bottom-right of those.
Sheet 3318CD is the Cape Town sheet. The code system means that if you know the latitude and longitude of a town, you can work out which map sheet to order without ever seeing the map.
1.3 Marginal information
Around the edge (the margin) of every topographic map you find the marginal information: the sheet name (for example 3318CD CAPE TOWN), the scale written as a ratio and drawn as a line scale, the key (legend) explaining every symbol, the year of the survey, and the north arrow with magnetic information. Good map readers always check the marginal information first: the key on the sheet itself is the final word on what a symbol means.
1.4 Conventional signs: point, line and area symbols
Because a map is much smaller than the real world, features are shown with standard symbols called conventional signs. They come in three families:
- Point symbols mark a feature at a single spot: a single building (small black square), a windpump, a lighthouse, or a trigonometrical station (a small triangle with a dot, used by surveyors, with its exact height printed next to it).
- Line symbols show features that stretch in a line: roads (red), railway lines (black with cross-ticks), rivers (blue), power lines and hiking trails.
- Area symbols cover the whole patch of ground where something is found: cultivated land, orchards and vineyards (rows of green dots), pine plantations, vleis and dams.
1.5 Colours on a topographic map
Colour is itself a code. Learn this table and half of any map extract reads itself:
| Colour | Used for | Examples |
|---|---|---|
| Blue | Water | Rivers, dams, canals, vleis, the sea |
| Green | Vegetation | Orchards, plantations, golf courses, nature reserves |
| Brown | Relief (the shape of the land) | Contour lines, sand, dongas |
| Red | Main transport and built-up areas | Tarred roads, national routes |
| Black | Construction and names | Buildings, railways, boundaries, lettering |
1.6 Reading a map extract
The fictional extract below uses an alphanumeric grid: columns are lettered A to D across the top and rows are numbered 1 to 3 down the side. A block reference names the column first, then the row, so the dam sits in block B3. (In Chapter 3 you will upgrade to true coordinates in degrees and minutes.)
Worked example: reading the extract step by step.
- The triangle with the dot in block D1 is a trigonometrical station. The number tells us this hilltop is exactly 1264 m above sea level.
- The brown contour lines around it are labelled 1200 and 1220, confirming that the land in the north-east is high ground.
- The blue line is the Klip River. Rivers always flow from high ground to low ground, so it flows from D1 in the north-east towards the south-west, where it fills the dam in block B3 before continuing into A3.
- The green dots in block A2 are an area symbol for an orchard.
- The black squares in block C2 are buildings: the small settlement of Vlakfontein sits beside the red road, with the railway line passing just south of it.
Notice how much you can already say about a place you have never visited: its height, where its water comes from, what is farmed there and how to travel through it. That is the power of a topographic map.
Summary
- A topographic map shows natural and constructed features of a small area in detail; the South African series uses a scale of 1:50 000 (1 cm = 500 m).
- The map code (such as 3318CD) locates the sheet by degrees of latitude and longitude plus quarter letters.
- Marginal information includes the sheet name, scale, key and north arrow.
- Conventional signs are point, line or area symbols; colours follow a fixed code: blue = water, green = vegetation, brown = relief, red = main roads, black = construction.
- A block reference such as B3 names the column letter first, then the row number.
Key terms: topographic map; scale; map code; marginal information; key (legend); conventional signs; point, line and area symbols; trigonometrical station; contour line; block reference.