Map Work and GIS

Why map work?

Maps are scaled drawings of the earth's surface. Kenya uses Survey of Kenya topographic maps (for example 1:50,000 sheets) for planning roads, farms and settlements.

Map scale

Expressed three ways:

1. Ratio/representative fraction: 1:50,000
2. Statement: 1 cm represents 0.5 km
3. Linear: a ruler line divided into km

Worked example: A road measures 6.2 cm on a 1:50,000 map. Actual distance = 6.2 x 0.5 = 3.1 km.

Direction and bearing

Direction uses the compass points. A bearing is measured clockwise from north (0 to 360 degrees). A town due east has a bearing of 090 degrees.

Grid references

The map is covered by a square grid. Four-figure references name a grid square; six-figure references locate a point to 100 m. Always read eastings first, then northings.

Representing relief

• Contours: close lines mean steep slope; wide apart mean gentle slope.
• Spot heights and trigonometrical stations.
• Form lines (dashed) and hill shading.
• Features: ridge, valley, spur, escarpment, plateau.

Cross-sections and gradient: gradient = vertical interval / horizontal distance. Vertical exaggeration = vertical scale / horizontal scale.

Area calculation

• Regular shapes: use geometry.
• Irregular shapes: count full squares plus half of the partial ones; each square on a 1:50,000 map is 1 km2.

Map interpretation

Describe relief, drainage (rivers, lakes), vegetation, settlement patterns (nucleated, linear, dispersed), transport networks, and economic activities (farming, trading). For example, a map of Kisumu shows a lake port, a railway terminus and a dense settlement on flat land.

Photographs and fieldwork

Aerial photographs and satellite images supplement maps. Fieldwork steps: statement of the problem, hypotheses, data collection (questionnaire, observation, sketching), analysis, report.

Geographic Information Systems

GIS stores spatial data in layers and links it to attribute data. Components: hardware, software (QGIS, ArcGIS), data, people, methods.

Data types: vector (points, lines, polygons) and raster (pixel grids).

Uses in Kenya:

• Mapping land use and parcels (Ardhi Sasa).
• Tracking drought and food security using satellite data.
• Planning roads and water supply.
• Monitoring wildlife in Maasai Mara and Tsavo.
• Disaster management for floods in Budalangi.

GPS gives location using satellites, and remote sensing collects data from a distance.

Try it

• Use a 1:50,000 sheet to give a six-figure reference of your school.
• Calculate the gradient between two points 800 m apart and 40 m in height difference.
• List three layers you would include in a GIS map of a flood-prone area.