MIDLANDS STATE UNIVERSITY
DEPARTMENT OF SURVEYING AND GEOMATICS
SVG406 DIGITAL CARTOGRAPHY
LECTURE NOTES
PREPARED BY
D NJIKE
2006
Introduction
This module is given to Level II students of the Bachelor of Science Surveying and Geomatics Honours degree programme. The prerequisite for this module is a pass in Cartography II in Level II and the module weighting is one examination is by a 3-hour paper held in each semester. Module marks are compiled during the session and based on practical work and marked assignments as well as test. Final assessment is based on 30%of the module marks and 70% of the examination marks. Two 2-hour lectures and one 2-hour practical per week are given in the first semester – making a total of 44 hours.
Aims and Objectives:
The aim is to provide the basic principles of digital cartography, to master the different hardware devices used to acquire digital data from hardcopy maps, to master the different algorithms used to develop digital cartographic packages and to look at how such systems have been implemented in different organisations.
The module is covered under the following topic headings:
1. Digital Mapping Hardware Devices
2. Digital Mapping Algorithms
3. Automated Cartographic Processes
4. Digital Terrain Modelling (DTM)
5. Digital Mapping Packages
The objectives within each topic are given below. At the conclusion of each section within a topic a student is expected to have understood the material covered. At the conclusion of the module the student is expected to be able to
i) understand the design and construction of different digital mapping input and output devices as well as the software associated with a digital mapping system.
ii) Analyse and design digital mapping systems.
The module objectives are:
1. To be able to understand the hardware composition of a digitiser, scanner, plotters.
2. To be able to understand the different visual displays, colour screens, IBM standards.
3. To know different transformations available in digital mapping packages.
4. To be able to program simple graphics routines like Bresenham’s line algorithm.
5. To know automation of cartographic problems e.g. generalisation, hill shading etc.
6. To be able to know the different digital mapping solutions in some organisations.
7. To know digital terrain modelling.
8. To be able to establish a digital mapping production flow line.
Detailed Module Outline
1. Introduction
1.1. What is Digital Cartography
1.2. Justification of Digital Cartography
1.3. Applications of Digital Cartography
1.4. Consequences of Digital Cartography
2. Data Acquisition Methods for Digital Mapping
2.1. Ground survey methods
2.2. Aerial survey methods
2.3. Graphics digitizing methods
3. Digital Mapping Hardware
3.1. Tablet and Solid State Digitisers.
3.2. Automatic Line Following Devices
3.3. Scanners
3.3.1. Drum scanners
3.3.2. Raster scanners
3.4. IBM standards
4. Cartographic Display and Plotter Technology
4.1. Graphic Display devices
4.1.1. CRT
4.1.2. Vector refresh displays
4.1.3. Vector storage displays
4.1.4. Raster refresh displays
4.1.5. Raster storage displays
4.2. Hard Copy output devices (plotters)
4.2.1. Vector Flatbed plotters
4.2.2. Vector drum plotters
4.2.3. Raster flatbed plotters
4.2.4. Raster drum plotters
5. Digital Mapping Algorithm (10hrs)
5.1. Drawing Algorithms
5.1.1. Bresenham Line drawing
5.1.2. Circle drawing
5.2. Clipping Algorithms
5.2.1. The Cohen-Sutherland Line clipping algorithms
5.2.2. Sutherland-Hodgman Polygon Clipping Algorithm
5.3. Transformations
6. Automated Cartographic Processes
6.1. Generalisation
6.1.1. Elements of generalisation
6.1.2. Line Simplification algorithms
6.1.3. Line Smoothing algorithms
6.2. Automatic Name Placement
6.3. Automated Relief portrayal
6.3.1. Relating Topography and Thematic Information
6.3.2. Shading
6.3.3. Antialiasing
6.3.4. Surface Representation
6.4. Digital Mapping Production Flowline
7. Digital Terrain Modelling
7.1. Data Acquisition
7.2. Measurement Patterns
7.3. Modelling Techniques
7.4. 3D Visualisation(Rendering) Techniques
7.5. Software and Applications
8. Digital Mapping Packages
8.1. Device Independent Software
8.2. Graphic Primitives (Graphic Kernel System)
8.3. Geographic Information Systems
8.4. Digital Map Production Examples
8.4.1. Ordnance Survey of Great Britain Production Flowline
8.4.2. Zimbabwean Department of the Surveyor General
8.4.3. Zimbabwean Department of Geological Survey
9. Trends and Concerns in Digital Cartography
1. INTRODUCTION
1.1. What is Digital Cartography
The application of computers and computer controlled devices in the capture, processing and output of graphic (map data).
1.2. Justification of Digital Cartography
· Speed up map production
· Reduce tedious cartography processes e.g. scribing, draughting, mask cutting, lettering etc.
· Cope with faster new data capturing techniques e.g. Remote Sensing, Photogrammetry, GPS.
· Ever increasing demand for up to date maps
· Data storage - high
· Greater flexibility using mapping
· Long run – cheap
· Increased accuracy
1.3. Applications of Digital Cartography
· National mapping – derivation of maps 1:1250 1:2500 1:10000 1:25000 1:50000
· Cartographic production of maps
· Multi – coloured maps
· Cartometrics - easy measurements and more accurate
· Data banks and information systems – conversions
· Terrain Modelling (map 2d
· (DTM - 2½D)
· Air craft simulator
· Guiding misks
1.4. Consequences of Digital Cartography
· Large capital investment – expensive devices
· High technical environment and suitable infrastructure - power supply, good communication networks.
· Skilled manpower
o Data Capture (Analogue to digital form, co-ordinates)
o Data processing
§ co-ords – transformations
§ scale
§ editing – (correct errors)
· Data Output
· Digital – analogue (Back ) (vector : Raster) Rasterisation, Vectorisation
· Capture
· 1st stage in map production othorphotos, manuscripts
· Conversion from analogue- digital (digitalisation)
2. DATA ACQUISITION METHODS FOR DIGITAL MAPPING
There are three main methods by which data can be acquired for input to a digital mapping system. These are:
(i) Ground survey methods
(ii) Aerial survey methods
(iii) Graphics digitising methods
Table 2.1 set out the main characteristics of each of these methods of data acquisition for a digital mapping system.
2.1. Ground survey methods
· Based on the use of electronic tacheometers, total stations, GPS etc
· Very accurate data but slow
2.2. Aerial survey methods
· Based on the use of stereo-plotting instruments
· Use of Remote Sensing imagery
The two methods above will be covered in more detail in other modules.
Source of digital mapping data / Instrumentation and methods used / Accuracy of digitised data / Aerial coverage / Typical applicationsField survey / Electronic tacheometry using total stations equipped with digital data collectors / Very high
Measurements made in static mode only / Limited to specific sites of small aerial extent / Small area site planning and design – housing estates, individual large builidings, road improvement schemes
Photogrammetric measurements / Analogue or analytical stereo-plotting machines equipped with encoders and recorders. 3D coordinates (XYZ) or (ENH) are recorded digitally / High - if discrete point measurements made in static mode
Low – continuous plotting of line detail and continuous carried out in a dynamic mode / Larger are projects, especially in rough terrain / Large engineering projects of considerable aerial extent – dams, reservoirs, roads, railways, open cast mining
Existing survey plans and topographic maps / Manual point /line following digitising
Semi-automatic line following digitising
Fully automatic raster scan digitising / Lower – quality dependent on the scale and accuracy of exiting plan or map.
Field survey may be necessary for map revision/updating
Supplementation of height and contour information may be required – carried out by field photogrammetric methods / From small areas covered by large scale maps to extensive areas usually covered by maps at medium scales / Preliminary planning and design based on exisiting plans and maps. Also landscape representation and visualisation e.g. for environmental impact studies
2.3. Graphics digitizing methods
Although modern surveying methods are able to produce their results directly in a digital format, the bulk of the available topographic surveys were stored originally on conventional map documents which must be converted to digital form to be used for GIS and related applications.
Digitising - secondary data acquisition
(Survey - primary acquisition)
Manual Digitising
- most widely used
- digitising systems usually include facilities for recording both spatial and non spatial information
- Non spatial – attribute codes
Types of digitising
Blind digitising –no display of data
Interactive digitising – data is displayed as it is digitised, can correct for mistakes
Types of cursors
· mouse
· Pen –type (stylus )
· 3 to 25 button cursors
Cartographic digitising modes
· Point digitising
· Stream digitising
- Point digitising
Cut amount of data
2. Stream digitising
Automatic; not the decision of operator
(a) Time based – huge amounts of data because cursor extracts information every second.
(b) Distance based – e.g. a centimetre – problem of how to pick out the distance- You can have redundant or less data
(c) Grid Based - extracts information at every grid crossing. Grid should be of the right grid size.
Feature codes – feature code list
(a) verbal/descriptive – easy to recognise but shown and cumbersome
(b) alphabetic – A,B,C…., simple entry – limited and illogical
(c) alphanumeric – A1,A2, allow classification of features
(d) Numeric – Limited – so can use decimals 1 roads
1.1 country roads
1.2 provincial
Online – cumbersome e.g. blind digitising
Offline – via a program
Automatic Line following (Digitising)
(a) mechanically based
(b) non-mechanically based
Automatic Raster Scanning
(a) Drum scanners
(b) Flatbed scanners
(c) Cameras using linear array CCD – charge coupled device
Methods of Digitising
Features / Manual / AutomationLine following / selective
less data measured and recorded
time x to line length / Low speed
Less expensive devices
Feature coding (F.C)
Low speed data recording / High speed
Expensive
Operator
Intervention
Raster Scanning / Non-selective
Huge amount of data
Time x to length of document / Not practical / Very H. speed
Expensive
F.C only at post processing
Extensive post processing
3. DIGITAL MAPPING HARDWARE
Basically there are two main methods by which the line data on a map may be measured and digitized. These are:
(i) line following, and
(ii) (ii) raster scanning.
Combining these with the two possibilities of manual and automatic or semiautomatic operation gives four possible techniques for the digitizing of line data on plans and maps.
It will be seen from these characteristics that only three of the four possible methods of digitizing the detail on existing topographic maps can be implemented in practice. These are:
(i) Manual point and line-following methods of digitizing
(ii) Automatic or semi-automatic line-following digitizing methods
(iii) Automatic raster-scan digitizing of complete map sheets.
3.1. Manual Digitizers (Tablet and Solid State Digitisers).
A very large number of manually-operated digitizers are available on the market, but these may conveniently be regarded as falling into one of two classes: mechanically-based digitizers or tablet digitizers.
Main Components
1) flat surface - size range 30 x 30 cm to 120 x 80 cm or more in dimension.
2) Handheld puck or cursor is to indicate positions to be recorded.
3) A keyboard for entering alphameric data and possibly commands
Larger devices mostly in use
- Exact positioning of puck is made possible by a cross hair mounted within a flat glass panel, which may sometimes include a magnifying lens.
- Also mounted on the puck are buttons that may be used for controlling data entry.
- The most commonly used technology for the digitising tablets is electromagnetic in which a table inlaid a fine grid of wires is associated with the puck which contains a metal coil.
- the grid of wires in the table and the coil is the puck art either as transmitter and receiver, or receiver and transmitter respectively.
- if the puck is the transmitter, the position of the cross hair is found by scanning the X and Y co-ordinate grid wires to identify those nearest to the puck.
- The exact position is found by interpretation between the adjacent wires on the basis of the nature of signals received.
- Smaller format digitising tablets may sometimes use a stylus with a small cool in its tip, rather than a puck as the locating device.
Operation of manual Digitisers
Starts by taping the source dement firmly to a digitising surface
Grid or graduate reference points may then be digitising in order to register the map’s coordinate system, before going on to digitise the map features.
Essential
- is to ensure that the locational refreshing information, represented by grids and graphic Graticules on the map, is retained in the digital version.
3.1.1. Mechanically-based Manual Digitizers
· utilize mechanical slides equipped with a measuring cursor
· have linear or rotary encoders to generate the rectangular (x,y) coordinate positions of the planimetric and contour data
- Encoders - transforms analogue data to digital
· The operator then carried out the measurement of the positions of individual point features using a cursor equipped with a measuring mark.
· Digitizing was done in either point or stream mode.
· Pencil Follower – a special type of mechanically-based digitizer employing cross-slides which was widely used for digitizing data on existing maps in the late 1970s
· the cross-slide and its supporting rails were positioned below the surface on which the map was placed.
· The measurement of the line detail was carried out by the operator using a cursor equipped with cross-hairs, around which a field coil was placed.
· This generated an electric field which was picked up by sensors (pick-up coils) mounted on a trolley, which in turn was mounted and could move along the cross-slide.
· When the cursor was located exactly above the trolley, the signals received by the opposite coils were equal, since they were in balance.
· As the cursor was moved by the operator to follow the line features, the signals detected by the pick-up coils went out of balance.
· This signal then activated a motor which moved the trolley via a pulley and wire system, with virtually no delay, so that it was again stationed below the new position of the cursor.
· A rotary encoder mounted on the other pulley wheel generated the coordinate position which was passed to the output electronics.