Cartography is fundamental to land survey professionals in Kenya, providing the tools and methods to represent the Earth's curved surface on flat maps accurately. Understanding map projection principles enables surveyors to select and apply appropriate projections that minimize distortion for specific geographic areas and purposes. This chapter focuses on identifying key geographic areas relevant to cartographic maps and selecting suitable map projection types, essential skills for producing reliable maps for land management, urban planning, and infrastructure development.

2.1 Cartographic map geographic area is identification.

In land surveying, the geographic area covered by a map significantly influences the choice of projection because distortions vary with location and extent. Kenyan surveyors must understand how projections behave over different geographic extents, whether local, regional, or global, to ensure precise measurements and spatial analysis. This topic explores how the location and extent of the area to be mapped guide the surveyor towards a suitable projection.

Meaning of geographic area identification

Geographic area identification is the process of defining exactly which part of the earth's surface a map will cover: where it lies, how far it extends, what shape it has and where its boundaries run. It is the first decision in any mapping project, because the location and size of the area control the map scale, the sheet layout, the data to be collected and the amount of distortion that must be managed. The area is usually defined by its boundary and by the latitudes and longitudes, or grid coordinates, of its extreme points. Example: before mapping a new irrigation scheme for a large farm in Kirinyaga, the survey team records the farm boundary beacons and the northernmost, southernmost, easternmost and westernmost coordinates of the scheme.

Elements that define a geographic area

• Location: The position of the area on the earth, given by latitude and longitude or by grid coordinates; Kenya lies across the equator, roughly between 5° N and 4.7° S.
• Extent: The size of the area, measured as its north to south and east to west distances or as its total area in square kilometres or hectares.
• Shape and orientation: Whether the area is compact, long and narrow, or irregular, and whether it stretches mainly north to south or east to west.
• Boundaries: The lines that enclose the area, such as plot boundaries, county boundaries, rivers, roads or coordinate lines.
• Purpose and level of detail: What the map is for, which decides how much detail must be shown and therefore the scale.
• Reference system: The datum and coordinate system in which the area's positions are expressed.

Classes of geographic area by extent

Tools and data for identifying a geographic area

Steps for identifying a geographic area

1. State the purpose: Agree with the client what the map is for and what it must show, as this sets the required detail.
2. Locate the area: Find the area on topographic sheets, registry maps or satellite imagery.
3. Define the boundary: Decide the exact boundary, using beacons, administrative limits, natural features or coordinate lines.
4. Record extreme coordinates: Note the latitude and longitude, or grid coordinates, of the northernmost, southernmost, easternmost and westernmost points.
5. Measure the extent: Calculate the north to south and east to west distances and the total area.
6. Describe shape and orientation: Note whether the area is compact or elongated and in which direction it stretches.
7. Record the reference system: State the datum and coordinate system used for all coordinates.
8. Document the findings: Prepare a short area description with a location sketch for the project file.

Worked example: extent of a county survey area from coordinates

North to south distance = difference in latitude (degrees) × 111 km

East to west distance = difference in longitude (degrees) × 111.32 km × cos(mean latitude)

A county survey area extends from 0°10′ S to 0°50′ S and from 35°40′ E to 36°30′ E.

Difference in latitude = 50′ − 10′ = 40′ = 40 ÷ 60 = 0.667°

North to south distance = 0.667 × 111

North to south distance = 74.0 km

Difference in longitude = 36°30′ − 35°40′ = 50′ = 50 ÷ 60 = 0.833°

Mean latitude = 0°30′ S = 0.5°, and cos 0.5° = 0.99996

East to west distance = 0.833 × 111.32 × 0.99996

East to west distance = 92.7 km

Approximate area of the bounding rectangle = 74.0 × 92.7

Approximate area = 6 860 km²

Worked example: choosing a scale to fit the area on a sheet

Scale denominator = ground distance ÷ map distance (in the same units)

A road contractor wants a 22 km long road corridor to fit on a sheet with 0.80 m of usable length.

Ground distance = 22 km = 22 000 m

Scale denominator = 22 000 ÷ 0.80

Scale denominator = 27 500

The smallest standard scale that fits is 1:50 000; at 1:25 000 the corridor would need 22 000 ÷ 25 000 = 0.88 m, which is longer than the sheet.

Chapter Summary

This chapter explored the fundamental principles involved in applying map projection techniques within cartography. It began with the identification of geographic areas relevant to cartographic maps, emphasizing the importance of accurately defining spatial extents for effective map creation. The discussion then progressed to selecting appropriate map projection types, highlighting the distinct characteristics and applications of the Mercator projection and the World Geodetic System. Each projection type was examined in terms of how it preserves or distorts geographic properties such as shape, area, distance, and direction. Finally, the chapter addressed the overall concept of cartographic map projection, explaining how the three-dimensional earth surface is transformed onto a two-dimensional plane. This transformation is crucial for accurate spatial representation and practical map usage across various fields. Understanding these principles equips learners to make informed decisions when creating or interpreting maps for different geographic contexts.