Cartographic map production is a fundamental skill for land survey professionals in Kenya, enabling them to transform raw geospatial data into accurate, readable maps that support land management, planning, and development. This chapter explores how diverse raw data types are integrated and processed to produce reliable cartographic products. Understanding the nature and handling of base maps, coordinates, distances, aerial photographs, satellite imagery, bearings, and reduced levels is essential for delivering precise maps that comply with national standards and client requirements.

1.1 Raw data

Raw data in land surveying refers to the initial measurements and observations collected in the field or obtained from remote sensing platforms before any processing or interpretation. This raw data forms the foundation upon which maps are constructed, requiring careful handling and transformation to ensure spatial accuracy and usability. In Kenya, surveyors working for county governments, land registries, or private firms must handle various raw data types to produce cadastral and topographic maps that support land allocation, infrastructure development, and environmental management.

1.1.1 Base map

The base map serves as the foundational layer in cartographic production, providing essential geographic context and reference points for overlaying additional data. It typically includes features such as roads, rivers, boundaries, and landmarks, which are critical for orientation and spatial relationships.

Characteristics of a Base Map

• Accuracy: The base map must have high positional accuracy to serve as a reliable reference for subsequent mapping tasks. Inaccurate base maps can lead to errors in property boundaries and infrastructure placement.
• Scale Appropriateness: The scale of the base map should align with the intended map use; for cadastral mapping, large-scale base maps (e.g., 1:1,000) enable detailed representation.
• Topographic Detail: Inclusion of relevant natural and man-made features helps in contextualizing survey data, such as rivers, contours, and roads.
• Up-to-date Information: Regular updating is essential, especially in rapidly changing urban or rural areas, to reflect new developments or landscape changes.
• Standardization: Use of standardized symbols, colors, and conventions ensures consistency and ease of interpretation across different maps and users.

Sources of Base Maps

Base maps in Kenya are often sourced from government agencies like the Survey of Kenya or downloaded from platforms such as the Kenya Open Data Portal. Surveyors may also generate their own base maps from previous surveys or digitize paper maps for integration with new data.

Role in Land Surveying

Base maps provide the spatial framework for plotting survey points, boundaries, and features. For example, in land adjudication projects, base maps assist in overlaying parcel boundaries for verification against field measurements.

Preparation and Digitization

Digitizing paper base maps involves scanning and georeferencing them to coordinate systems such as UTM Zone 37S used in Kenya. This process ensures compatibility with GPS and GIS datasets.

1.1.2 Coordinates

Coordinates with oral refer to the spatial positioning data obtained and communicated during field surveys, often involving verbal transmission of coordinate values. This practice is especially relevant in collaborative surveying exercises or when relaying data between field teams and office personnel.

Meaning and Use of Coordinates in Surveying

Coordinates denote the precise location of points on the earth's surface, usually expressed in a coordinate system such as UTM or geographic latitude and longitude. They enable accurate plotting of land features and boundaries on maps.

Oral Communication in Field Data Collection

In many Kenyan survey projects, especially in remote or informal settlements, surveyors rely on oral communication to share coordinate data due to limited digital infrastructure or challenging terrain. This requires clarity, accuracy, and verification to avoid transcription errors.

Methods to Ensure Accuracy in Oral Transmission

• Repetition: Repeating coordinate values aloud ensures the receiver correctly records the data.
• Phonetic Clarity: Using clear pronunciation of numbers and coordinate components reduces misunderstanding.
• Cross-Verification: Comparing coordinates with GPS device readings or printed maps confirms correctness.
• Use of Reference Points: Referring to known landmarks helps anchor coordinate data during oral transfer.
• Documentation: Immediate recording of orally transmitted coordinates in field notebooks or digital devices prevents data loss.

Challenges and Mitigation

Noise, language barriers, and unclear articulation can lead to errors. Employing standardized coordinate formats and training field teams in communication protocols improves reliability.

1.1.3 Distances

Distances represent the linear measurements between two points on the ground, fundamental for establishing boundaries and features in land surveying. Accurate distance measurement underpins the precision of maps and cadastral records.

Measurement Techniques

Distances are measured using a variety of instruments including tape measures, electronic distance measurement (EDM) devices, and total stations. Each technique offers different levels of accuracy and efficiency suited to specific survey contexts.

Horizontal vs. Slope Distances

Figure: slope distance.

• Horizontal Distance (H): The projection of the distance between two points onto a horizontal plane. This is the distance plotted on a map, because a map is a horizontal plan.
• Slope Distance (S): The actual distance measured along the ground between the two points. It is always longer than the horizontal distance unless the ground is level.
• Height Difference (V): The vertical distance between the two points, used in computing levels.

Correction of Distances

In Kenyan terrain, slope corrections are necessary where steep gradients occur. Where the slope angle θ is measured:

• Horizontal Distance = Slope Distance × cos(θ)
• Height Difference = Slope Distance × sin(θ)

Example. A slope distance of 100.00 m is measured along ground rising at 10°.

• H = 100.00 × cos 10° = 100.00 × 0.9848 = 98.48 m
• V = 100.00 × sin 10° = 100.00 × 0.1736 = 17.36 m

Plotting the 100.00 m directly would place point B 1.52 m too far away on the map.

Units of Measurement

Distances are typically recorded in meters in Kenya, aligning with the metric system used in surveying standards. Consistent units prevent errors in data integration and map scaling.

Importance in Cadastral Surveys

Precise distance measurements define property boundaries clearly, preventing disputes and facilitating legal land transactions.

1.1.4 Aerial photographs

Aerial photographs are images captured from aircraft or drones, providing a bird's-eye view of the terrain. They are invaluable in mapping large areas quickly and detecting features not easily accessible on foot.

*Figure: aerial photographs.

Types of Aerial Photographs

• Vertical Photographs: Taken with the camera axis perpendicular to the ground. They cover a near-rectangular area at a nearly constant scale, which makes them suitable for planimetric mapping.
• Oblique Photographs: Taken with the camera axis tilted. They cover a trapezium-shaped area whose scale changes across the photograph, so they are used for visual interpretation rather than measurement.

Interpretation and Application

Surveyors use aerial photographs to identify landforms, vegetation, water bodies, and infrastructure. For example, in agricultural cooperatives, aerial photos help map farm boundaries and crop patterns.

Scale and Resolution

The scale of a vertical photograph depends on the focal length of the camera and the height of the camera above the ground:

Scale = f ÷ (H − h)

where f is the focal length of the camera, H the flying height above datum, and h the height of the ground above datum. Both must be in the same units.

Example. A camera of focal length 152 mm is flown at 2020 m above datum over ground at 500 m above datum.

• H − h = 2020 − 500 = 1520 m = 1 520 000 mm
• Scale = 152 ÷ 1 520 000 = 1 : 10 000

Because h varies with the terrain, the scale of a photograph over hilly ground varies from point to point. Higher resolution images support detailed feature extraction.

Photogrammetry

Using stereo pairs of aerial photographs, surveyors can derive three-dimensional information such as contours and elevations, enhancing map accuracy.

Limitations and Weather Dependence

Cloud cover, shadows, and seasonal changes affect photo clarity. Surveyors plan aerial photography missions during optimal weather conditions to ensure quality data capture.

1.1.5 Satellite imagery

Satellite imagery offers extensive spatial coverage and frequent updates, making it a powerful tool for land surveyors in Kenya. It complements aerial photographs by covering inaccessible areas and providing multispectral data.

Types of Satellite Images

• Multispectral Images: Capture data in multiple wavelengths, enabling vegetation and land use analysis.
• Panchromatic Images: Black-and-white images with high spatial resolution, useful for detailed mapping.

Sources of Satellite Data

Platforms such as Sentinel, Landsat, and commercial providers like Maxar (formerly DigitalGlobe) supply imagery accessible to Kenyan surveyors. Government agencies also provide free or subsidized data for development projects.

Image Processing

Techniques such as image enhancement, classification, and georeferencing prepare satellite images for integration into GIS and cartographic products.

Applications in Land Surveying

Satellite imagery supports land cover mapping, change detection, and environmental monitoring. County governments use it to update cadastral boundaries and monitor urban expansion.

Challenges of Satellite Imagery

Cloud cover and atmospheric disturbances can obscure features. Additionally, the spatial resolution may limit detail for small-scale surveys, requiring integration with ground data.

1.1.6 Bearings

Bearings indicate the direction from one point to another, expressed as angles relative to a reference direction, usually north. They are critical in defining boundary orientations and navigating survey traverses.

Figure: bearings three norths.

Types of Bearings

• True Bearings: Measured relative to geographic (true) north, the direction of the North Pole, offering a consistent reference.
• Magnetic Bearings: Measured relative to magnetic north, the direction a compass needle points, affected by local magnetic declination.
• Grid Bearings: Measured relative to grid north, the direction of the vertical grid lines on a map projection, used in large-scale mapping.

Whole Circle Bearings

A whole circle bearing is measured clockwise from north, from 0° to 360°. North is 0°, east 90°, south 180° and west 270°. The back bearing, the direction from B back to O, differs by exactly 180°: add 180° if the bearing is less than 180°, subtract 180° if it is more.

Measurement Methods

Surveyors use compasses, theodolites, or total stations to measure bearings accurately. Calibration against known reference points ensures precision.

Magnetic Declination and Grid Convergence

• Magnetic declination is the angle between true north and magnetic north. It varies across Kenya and changes slowly over time, so the current value for the area must be used. True bearing = magnetic bearing + declination where declination is east, or − declination where it is west.
• Grid convergence is the angle between true north and grid north. It is zero along the central meridian of the projection zone and increases away from it.

Use in Boundary Definition

Bearings combined with distances define property lines clearly in legal descriptions and survey plans.

Recording and Reporting

Bearings are recorded in degrees, minutes, and seconds, and must be consistently reported in survey documentation, stating which north they refer to, to avoid ambiguity.

1.1.7 Reduced levels processing data

Reduced levels (RLs) represent the vertical height of points relative to a chosen datum, essential for representing terrain elevations and planning construction works.

Figure: levelling reduced levels.

Concept of Reduced Levels

RLs express the elevation of survey points after adjusting raw height measurements to a common reference, often mean sea level. Levelling always starts from a benchmark, a fixed point whose RL is already known.

Terms Used in Levelling

• Backsight (BS): The first staff reading taken after setting up the level, on a point of known RL.
• Foresight (FS): The last staff reading taken before moving the level, on a point whose RL is to be found.
• Intermediate sight (IS): Any reading between the backsight and foresight from the same setup.
• Line of collimation: The horizontal line of sight through the level.
• Height of collimation: The RL of the line of collimation.
• Change point: A point on which a foresight is taken, and then a backsight from the next setup, linking one setup to the next.

Methods of Determining RLs

• Height of Collimation Method, also called the height of instrument method: The RL of the line of sight is found first, then each reading is subtracted from it.
  • Height of collimation = RL of benchmark + BS
  • RL of any point = height of collimation − staff reading on that point
• Rise and Fall Method: The difference between consecutive staff readings gives the rise or fall from one point to the next.
  • Rise or fall = previous reading − current reading. A positive result is a rise, a negative result a fall.
  • RL of a point = RL of previous point + rise, or − fall

Example. A benchmark A has RL 1520.450 m. The backsight on A reads 1.235 m and the foresight on B reads 0.870 m.

• Height of collimation = 1520.450 + 1.235 = 1521.685 m
• RL of B = 1521.685 − 0.870 = 1520.815 m
• Check by rise and fall: 1.235 − 0.870 = 0.365 m rise, and 1520.450 + 0.365 = 1520.815 m

Processing Steps

• Collect raw staff readings, starting from a benchmark.
• Enter them in a level book in the order observed.
• Apply formulas to compute RLs relative to the datum.
• Apply the arithmetic check: ΣBS − ΣFS = last RL − first RL.
• Check for errors using closing checks in leveling loops, closing either back on the starting benchmark or on a second benchmark of known RL.

Application in Map Production

RL data enables the creation of contour lines and slope analysis on topographic maps, supporting infrastructure design such as road gradients.

Common Errors and Corrections

Instrument misleveling, staff reading mistakes, a staff not held vertical, and datum inconsistencies cause RL errors. Systematic checks and repeated measurements improve reliability. Keeping backsight and foresight distances roughly equal cancels most errors caused by the level not being perfectly adjusted.

Chapter Summary

This chapter explored the foundational elements of cartographic map production, beginning with the collection of raw data including base maps, coordinates, distances, aerial photographs, satellite imagery, bearings, and reduced levels processing data. The subsequent step involved processing this raw data to ensure accuracy and usability for mapping purposes. Georeferencing was then covered as a critical process to align spatial data with real-world coordinates, enabling precise map creation. Various types of maps and plans were examined, such as RIM, topographic, atlas, deed plans, thematic, geological, and cadastral maps, each serving unique purposes in spatial representation. The concept of map scale was detailed with distinctions among large, small, and medium scales, alongside methods of scale representation including graphical, representative fraction, and statement forms. Finally, the chapter addressed the production phase where all designed elements are compiled into the final map product ready for practical application. This comprehensive overview equips students with the principles necessary for effective cartographic map production.