Topographic/Cadastral maps

• Air photographs
• Remote sensors
• Field notes & coordinate lists
• Existing maps

Thematic maps

• Census reports
• Meteorological records
• Historical documents

Total stations

Photogrammetric workstations/plotters

Digitizers

Scanners

Integrate 3 devices

1. Functions of angle measurement
2. Distance measurement
3. Digital data storage

Photographs are obtained from wide angle aerial cameras

The key concept in photogrammetry is parallax which is the apparent displacement in the image of an observed object due to different points of view

The use of aerial photographs for mapping allows parallax measurements to be done in the office with the help of photogrammetric stereoplotters (analytical and digital) & computers

Use digitizing tables with translucent surfaces used as a screen for the projection of an image from behind

Converts existing maps into digital form

• Manual digitizers - very slow
• interactive (or semi-automated) line following system - intelligent cursor

Automated digitizers

Are based on the principle of breaking an image into pixels

Remote sensors

Satellite positioning systems

Aerial photography

Satellites

Provide extremely current data for thematic and topographic mapping at small scales

Sensors detect events, changes, and physical characteristics of a given area by transforming stimuli (sound, light, heat, or motion) into electrical signals

Sensors collect data about the Earth's surface

• Seisometers measure ground motion
• LIDAR 3D laser-based aerial mapping
• Sonar detects objects under water through sound propagation

GPS, GLONASS etc

In SPS the control stations are satellites whose positions in orbit can be determined accurately

Positional data is obtained by:

GPS receivers intercept satellite signals and perform trilateration

• Static method
• Kinematic method

Early attempts at aerial photography included balloons, kites, and even rockets

Modern aerial photography now relies on advanced technology like helicopters & unmanned aerial vehicles (UAVs) - well known as drones

Drones are able to reach impressive heights & are controlled by hand-held remote

Satellites serve a variety of purposes - from spying on foreign adversaries, to tracking weather and improving cell service, enabling the GPS network

Satellites enable consistent, large-scale updates of Earth's surface

Satellites continuously capture images of the Earth's surface, for maps - used in agriculture, utilities, forestry, earth sciences, global change, & regional planning

Preparation depends on data collection method or intended purpose

• Field survey (GPS or Total Stations) data for topographic or cadastral maps
• Field survey with UAVs for topographic survey, volume calculations or planning
• Digital photogrammetric methods

Field survey with controls does not require further spatial adjustments

Direct acquisition of 3D coordinates

Require data to be downloaded & exported to suitable format, then data cleaned of any unnecessary points picked during survey

File is saved in CAD suitable input file format

Minimum GCPs needed are 5 during fieldwork

RTK-GPS observations of the control points are undertaken

RTK-GPS data is incorporated using drone mapping system

Processing software products; Orthomosaic, Digital Surface Model (DSM)

Digital photogrammetric workstation is designed to perform measurement functions of an analytical plotter as follows:

• Integrate raster image and vector map data
• Digital stereocorrelation for procedures of inner, relative, absolute orientation
• Implement image processing procedures such as contrast enhancement, edge sharpening, vector on raster overlay
• Automatically generate digital elevation models (DEMs)
• Produce digital orthophotos
• Produce new type of cartographic production e.g. virtual reality scenes

Hardcopy maps & orthophoto are data entry sources for digital repositories

Analog to digital conversion methods

• Manual digitization
• Raster scan to vector digitization
• Stereo digitization

Digitization process

Operator intelligence is permitted in the process

This allows linking of attributes to the graphic element during digitization

Use of automatic codes, menu command tablets are powerful integral components of the digital graphic records

It is simple to separate & segregate different types of information

Raster scanning & vectorisation of contours

Height tagging & data editing

Geometric transformation & interpolation

Intercell processing, check plots

Quality control - checking for errors

Map production - topo map

Stereoscopy - overlapping Imagery acquired from different locations to produce a 3D model

Stereo mapping consists of preparing vector or topographic data from stereo aerial or satellite Images based on specification

Specification for mapping of topographic and thematic features depends on the client's requirements

Applications of photogrammetric stereo techniques are; identification, measurement, & manual digitization of 3D features from a triangulated stereo model

Map abstraction is the process of representing entities on a map

Involves 5 steps

1. selection
2. classification
3. simplification
4. exaggeration
5. symbolization

Map symbols are constructed in 2 ways:

1. Graphic variables
2. Color schemes

Queries about this Lesson, please send them to: jmwaura@jkuat.ac.ke

*References*

• Analytical and Computer Cartography, 2nd ed. Keith C. Claike
• Geographic Information Systems: The Microcomputer and Modern Cartography, 1st ed. Fraser Taylor