Before thematic or marine resource mapping can proceed it is essential to understand the content and construction of base maps which constitute the foundation on which thematic information is overlaid. Base maps are generally in the form of topographic maps or hydrographic charts.
The topographic map presents the cultural and physical features of the land, usually in various colours, and gives their exact geographic location in terms of latitude, longitude and elevation above sea level. Features on a topographic map may be classified into four main divisions: water, relief, culture and vegetation (refer to Section 1).
Three methods are commonly used to express scale on maps (refer to Section 3). They may be summarized as follows:
i) representative fraction, which is the proportion of map distance to true distance expressed as a mathematical ratio, e.g., 1:250,000;
ii) scale statement, which is a written statement of map distance in relation to true distance, e.g., 1 centimetre = 10 kilometres;
iii) bar scale, which consists of one or more graduated straight lines which are subdivided into units of ground distance.
Topographic maps are commonly produced in one of five scales:
|i)||1:25,000||-||4 centimetres to 1 kilometre or approximately 2 1/2 inches to the mile;|
|ii)||1:50,000||-||2 centimetres to 1 kilometre or approximately 1 1/4 inches to the mile;|
|iii)||1:125,000||-||1 centimetre to 1 1/4 kilometres or approximately 1/2 inch to the mile;|
|iv)||1:250,000||-||1 centimetre to 2 1/2 kilometres or approximately 1/4 inch to the mile;|
|v)||1:1,000,000||-||1 centimetre to 10 kilometres or approximately 1/16 inch to the mile.|
Many countries, however, still use older mapping at 1:63,360 or 1 inch to 1 mile, and 1:15,840 or 1 inch to 1/4 mile.
In Canada, for example, the National Topographic Series uses the Transverse Mercator projection as the geographical coordinate system of latitudes and longitudes on which is superimposed the UTM Grid System; Canada is divided into sixteen North-South strips or zones of 6° longitude (refer to Section 4.3 and Figure 5.1). These strips are part of the world-wide UTM Grid System of 60 zones.
The key stages of topographic map production are as follows:
An aircraft equipped with a special aerial survey camera, flying at a specific speed and an altitude of 5,000 to 10,000 metres, takes a succession of photographs of the terrain in parallel strips with 20-40% overlap laterally and 60% in the direction of flight (Figure 5.2). Thus each portion of land appears at least twice in the photo series, a necessity for producing a three-dimensional visual model (image) from which the map information is derived. Aerial photography is repeated regularly to provide up-to-date information for the revision of topographical mapping.
An aerial photograph provides only a visual two-dimensional representation of the terrain. It does not show the height of mountains, depths of valleys, etc. The following are the procedures required to establish the exact geographic location of each photograph so that the features of the photograph can be accurately transferred to a map:
i) each photograph is given a grid to permit interrelation with adjacent photographs;
ii) field surveys establish primary control points by measuring their exact latitude, longitude and height above sea level.
Primary control points established by surveyors in the field include the following two types:
i) vertical control points, or bench marks, which provide a precise height above sea level;
ii) horizontal control points which provide precise latitude and longitude coordinates.
The primary control points are marked with a white cross and photographed by an aircraft flying directly above at a predetermined altitude. This operation ties some of the aerial photographs to a precise ground location, a primary control point. Approximately one in ten aerial photographs is positioned by this method.
Figure 5.1 UTM zones and central meridians for Canada. (After Canada, Department of Energy, Mines and Resources, 1976)
Figure 5.2 Lateral and forward overlap of aerial photographs. (After Canada, Department of Energy, Mines and Resources, 1976)
Figure 5.3 Photogrammetry: the mapmaker viewing two overlapping photographs stereoscopically to produce a three-dimensional model. (After Canada, Department of Energy, Mines and Resources, 1976)
Secondary control points, such as buildings, river junctions, headlands, etc., are chosen to fix the position of the remaining photographs. These control points, identified by surveyors in the field, may include the following two types:
i) tie points, used to mathematically tie together the adjacent photographs;
ii) pass points, which are only used in aerotriangulation to assist in the mathematical adjustment of measurements for the whole area.
Measurement of latitude, longitude, and height above sea level for the secondary control points is established by aerotriangulation. In this system the grid of the aerial photograph is used to provide grid coordinates for the secondary control points that occur in the photograph. With the aid of a computer and coordinate geometry, the individual grids of each aerial photograph are combined into one common grid for the whole area to be mapped. The latitude, longitude and elevation for the secondary control points are then calculated, based on their position relative to each other and the precisely surveyed primary control points.
Photogrammetry is a process by which information is transferred from the aerial photographs to the map manuscript. The process consists of a number of phases:
i) The photogrammetric compiler first produces a three-dimensional visual model by viewing two overlapping aerial photographs of the same area taken from different camera stations in a stereoscopic plotting machine (Figure 5.3). The model is positioned and expanded or contracted to fit the required scale according to the secondary control points, i.e. the two aerial photographs in the stereoscope are levelled to correct horizontal or vertical displacement due to the tilt or heading of the photographic aircraft;
ii) A manuscript is produced by tracing the required features on a sheet of translucent plastic. The photogrammetric equipment has a small floating mark in its viewer that the compiler moves throughout the model to trace any feature. A drawing stylus on a connecting drafting table follows the movement of the floating mark and sketches fine lines across the manuscript. For contour lines, the compiler sets the vertical height reading of the equipment at a selected elevation. The mark is then moved through the visual three-dimensional model so that it always appears to be in contact with the ground at that level, thus plotting a path of equal elevation, a contour line. The initial manuscript information shows cultural features, water, vegetation and contours. The information on the manuscript is verified by ground checking. Additional names, appropriate symbols, etc., are now added. The manuscript is then edited and inspected before the cartographic process begins.
This process transforms the initial manuscript information into separate negatives from which printing plates for each printing colour are made for map reproduction (Figure 5.4). Six basic colours are normally used in topographic map making:
Figure 5.4 Phases of topographic map production.(After Canada, Department of Energy, Mines and Resources, 1976)
|i)||black, for cultural features;|
|ii)||blue, for water systems and grid;|
|iii)||brown, for contours;|
|iv)||red/orange, for road systems;|
|v)||green, for vegetation.|
Three types of negatives are produced to make the printing plates for each colour:
i) Line negatives (refer to Section 11) : The manuscript base sheet is photographed or contracted to negative form. Using the negative, a guide image is reproduced on scribing film by a photo-chemical method. Scribing film is clear plastic with a coloured coating which is scribed or engraved (Figure 5.5) along the guide lines produced by the negatives on the coating. The end result is a line negative;
ii) Type overlay negatives (refer to Section 11): These show geographic names, labels, elevations, figures and bordering information which may appear on the final map in more than one colour (e.g., black for land features, blue for water features and brown for elevation numbers on contour lines). Separate type negatives are, therefore, produced for each colour. Each type item is given a type style and size. The type is positioned on a plastic overlay sheet in the place it will appear on the map, using a line positive of the base as an outline guide. The type overlay is then converted to a type negative by the contact process. For relatively simple jobs a single type overlay and single type negative are usually sufficient. This saves considerably on reproduction costs and also makes checking easier. Colour separation of the names is accomplished with masks;
iii) Area negatives (refer to Section 11): Normally called “peel coats”, these are also termed “open window” or “artificial” negatives. They are used to produced areas of solid or tinted colour, e.g., blue area for water. The linework image on a sheet of scribing film or line negative is etched photomechanically onto sensitized peelable material. The coating is then peeled from the area to be coloured or shaded, so that it becomes a clear plastic window.
Composite negatives for each of the printing colours are then produced. Alignment of the individual negative components is assured by punching registration holes in each negative which are fitted to metal pins or bars. The cartographer then prepares a colour proof (refer to Section 12.1.11) of the map, from all of the composite negatives, for editing purposes.
A colour plate for each of the printing colours is produced by exposing the image onto a light sensitive printing plate. This is achieved by shining an arc light through the composite negative for that particular colour in contract with the plate. The lacquered areas on the printing plate which show the map image retain ink and repel water; the non-image areas retain water and repel ink. The plate is pliable so that it can be shaped around a cylinder. Single paper sheets are then passed through the press and printed with the basic colour images and their tint variations (refer to Section 12).
Figure 5.5 Scribing. (After International Cartographic Association, 1984)
Figure 5.6 Map borders and margins. (After A.J Kers and P.J. Oxtoby, 1977)
All information that appears on a map can be digitized, i.e. converted into the computer's numerical code form. It can then be filed as computer data on magnetic tape or on hard or floppy disk for selection by the map maker. Digitized data can be fed into an automatic plotting machine to be reproduced as a map (refer to Section 14).
Information can be digitized directly from the photogrammetric plotter, thus avoiding the steps of preparing an initial map manuscript and manual digitization. After checking and editing, it can be automatically scribed on scribing film for the production of printing plates, or the information may be reproduced by a photo-head plotter on photographic film.
In addition to showing the features of part of the earth's surface within a given framework (graticule or grid), topographic maps contain marginal and border information. The type and position of this information has been standardized as follows (Figure 5.6):
i) Margin: the area of paper surrounding the outer framework of the map;
ii) Neat line: the line (graticule or grid) enclosing the mapped area;
iii) Border: the area between the neat line and the outer framework of the map;
iv) Map face: mapped area enclosed by the neat line.
The following is a list of items regarded as essential information for inclusion in the topographic map margin (Figure 5.7) and may be useful as a checklist when designing maps in general:
|i)||sheet name or title;|
|v)||identification panel (contains 2nd, 3rd and 4th items above)||;|
|vi)||date of aerial photography used for map compilation;|
|vii)||date of aerial photography used for map update;|
|viii)||area of coverage of the series (series title);|
|ix)||representative fraction scale;|
|x)||graphic bar scale;|
|xi)||unit of elevation used, e.g., metres or feet;|
|xii)||contour interval and unit, e.g., 2 metre contour interval;|
|xiii)||conventional signs (legend);|
|xiv)||elevation tint box;|
|xv)||index to adjoining sheets;|
|xvi)||notes concerning grid(s);|
|xvii)||instructions on the use of the grid reference system;|
|xviii)||declination diagram (information on the relationship between true, grid and magnetic north);|
|xix)||projection and datums;|
|xx)||names and boundaries disclaimer note, e.g., “This map is not an authority on international boundaries”;|
|xxi)||publication note, e.g., name of publishing agency;|
|xxii)||history note, e.g., type of production and list of sources on which map is based;|
|xxiv)||printing note and printer's imprint, i.e. distinct from publisher's name.|
The following is a list of items regarded as essential for inclusion as map border information (Figure 5.8):
|i)||geographical coordinates of the sheet corners;|
|ii)||values of graticule lines or ticks;|
|iv)||destination of road or railways;|
|v)||that portion of a name which overlaps into the next sheet.|
Optional marginal and border information may include the following:
|ii)||glossary of terms;|
|iv)||conversion diagrams, e.g., metres to feet;|
|vi)||representation of relief diagram;|
|vii)||other items which may be required in specific instances.|
A marine chart is essential for safe navigation and the practice of surveying and charting water for the purpose of navigation is known as hydrography. Marine charts are important to a number of economic sectors including:
|iv)||offshore and coastal oil and gas industry;|
|v)||coastal mines and industrial plants;|
Figure 5.7 Marginal information for a topographic map. (After J.S. Keates, 1973)
Figure 5.8 Border information for a ropographic map. (After J.S. Keates, 1973)
The fundamental operation in hydrographic surveying is sounding, that is, measuring water depths. These are indicated on a chart by the following conventions:
i) depth soundings represented by a point symbol with a number beside it indicating the depth;
ii) contour lines, referred to as bathymetric lines in marine charts which join soundings of equal depth;
iii) colour coding, which indicates an increase or decrease in depth by the use of varying tints of a colour.
For centuries soundings were obtained by a lead weight tied to a line lowered over the side of a ship. This method, while accurate, is time-consuming and does not give a continuous profile of the ocean. For detailed large-scale surveys of confined areas, and for shoal examination, however, the leading method is still used today.
Most modern surveys are carried out with an instrument known as Sonar (Sound Navigation and Ranging), also called an acoustic depth finder, echo sounder or fathometer. Depths are determined by measuring the time required for a sound wave to travel from a transducer mounted in the hull of a survey ship to the ocean bottom and back to the ship. The ocean bottom reflects sound as a mountain face reflects sound in air, producing an echo. In fact echos are more easily obtained from the sea bottom. Because of lower absorption, sound in water will travel many times as far as it will in air. The properties of sea water also ensure reasonably constant velocities of sound - about 1440 metres per second.
Sonar distances are obtained by measuring a signal's round trip travel time, dividing by two and multiplying by the velocity of sound in sea water. A measurement that previously took hours using the leadline method can now be obtained in seconds using sonar. Modern echo sounders continuously and automatically record signals, creating a continuous profile of the bottom relief along the ship's track.
Soundings must be located precisely on a chart. In former days when surveys were normally conducted within sight of shore, the geographic location of these soundings was measured with a sextant, an instrument for measuring angles. A typical sextant reading is obtained by simultaneously measuring two angles between three clearly marked “stations” on shore, the positions of which are known from previous measurements. From this, it is possible to plot the exact position of the vessel at the time the angles were measured (Figure 5.9 and Table 5.1). Today the sextant has been replaced with a number of electronic systems. With the exception of satellite systems there is an inevitable trade-off in positioning systems between range and accuracy. The systems developed for shorter range are normally more accurate than those used for long ranges.
Figure 5.9 Shipboard positioning. (After Canada, Department of Fisheries and Oceans, 1979)
|POSITION FIXING||Charts used for:|
|overseas navigation||coastal navigation|
|ocean||marginal sea||inland sea||offshore||inshore||approach||entrances channels||port|
|Detailed land topography||x||x|
|Landmarks visible from afar||x||x||x|
|All depth data||x||x||x|
|Nature of bottom and depth data for echo soundings||x||x||x|
|Nature of bottom||x||x||x|
|Selected marks on land or at sea||x||x|
|All marks on land or at sea||x||x||x||x||x|
|Radar conspicuous objects||x||x||x|
Selected maritime radio, radio navigation and radio determination stations
|Limits of radar stations||x||x||x|
|Hyperbolic navigation grids||x||x||x||x||x||x||x|
1. Soundings and depth contours must be selected so that their depiction permits the mariner to draw conclusions regarding the density of the surveys. Even very deep soundings must be shown on charts since areas without depth data will suggest incomplete surveys. The depth data must be shown for the whole area and not be limited to certain channels.
2. Coastal configuration and land topography are essential elements for position fixing and cannot be omitted, notwithstanding modern navigational methods. Topographic features extending inland from the coastal area may become necessary in the case of particularly conspicuous landmarks visible over a great distance, the depiction of spot heights alone being insufficient.
Some modern positioning systems include the following :
i) Short range systems (in sight of land) - microwave frequencies, e.g., Miniranger or Tellurometer MRD - accuracies of 10 metres, restricted to line of sight;
ii) Medium range systems - medium frequencies, e.g., Hi-Fix 6 or Argo - accuracies of 20–50 metres, 100–200 kilometres from shore;
iii) Long range systems - pulsed, low frequency:
a) LORAN-C is a pulsed, low-frequency, long range hyperbolic radio navigation system. It combines features of both LORAN-A and DECCA, two systems of navigation accepted throughout the world.
Hyperbolic navigation systems operate on the principle that the difference in time of arrival of signals from two stations, observed at a point in the coverage area, is a measure of the differences in distance from the point of observation to each of the stations.
LORAN-C stations are located on land and are grouped to form a “chain”; one station is labelled the Master (designated M) and the others are called secondary stations (designated W, X, Y, or Z). Signals transmitted from the secondaries are synchronized with the master signal.
As an example, in Figure 5.10, the master station (M) and the secondary station (X) transmit synchronized pulses at precise time intervals. The on-board LORAN-C receiver measures the slight difference in time that it takes for these pulsed signals to reach the ship from this pair of transmitters. The time difference (TD) is measured in microseconds, or millionths of a second, and is then displayed as one readout on the receiver. When at position “A”, the time difference displayed is 13,000.0 microseconds. This time difference can be plotted on a LORAN-C latticed chart on a line-of-position (LOP). With just this one number, the vessel could be located somewhere along the “13,000 line-of-position”.
Next a TD measurement is taken from the master station (M) and another secondary (in this case Y). The LORAN-C receiver then displays the TD between M and Y. Continuing with the same example, the TD displayed is 31,000.0 microseconds. Again the TD is plotted as a LOP and the vessel's position is located somewhere along the 31,000 LOP. Where the two LOP's intersect is the vessels exact location (position “A” in Figure 5.10).
b) The OMEGA system, developed by the U.S. Navy, provides world wide all-weather positioning of ships, aircraft and submarines (submerged) with a nominal accuracy of one mile in daytime and two miles at night. It is now widely used by non-naval vessels. OMEGA is a global system of eight landbased transmitting stations, so located that a user will receive signals from at least three stations. Any two signals can be used as a pair to establish a line-of-position (LOP).
Figure 5.10 A typical LOREN-C chain off the east coast of Canada. (Afetr Canada, Department of Transport, 1981)
As with LORAN-C, OMEGA is a very low frequency hyperbolic radio navigation system, but it uses phase difference measurements rather than a time-difference principle.
iv) Satellite systems (overlapping both medium and long range systems):
The Navy Navigation Satellite System (NAVSAT) was also designed for the U.S. Navy and was released for civilian use in 1967. It is suitable for any size of vessel, when it is economically justified, but the shipboard receivers and related equipment are considerably more expensive than other systems such as LORAN and OMEGA. The NAVSAT System consists of one or more satellites, each in a circular polar orbit at an altitude of about 1100 km. Only one satellite is used at any given time to determine position. The apparent change in frequency of the radio waves received when the distance between the source of radiation (Satellite) and the receiving station (ship, aircraft, submarine, etc.) is increasing or decreasing is termed the Doppler Shift. By means of this phenomenon, it is possible to calculate the location of the receiver on earth to within 10 metres if the satellite orbits are known, together with the speed and direction of the surface vessel.
Accurately positioned soundings must be specified on a chart as the depths below a reference level (chart datum, refer to Section 4). Selection of a suitable chart datum depends on a number of factors, including a detailed knowledge of past fluctuations in water levels, hence the installation of permanent and temporary water level gauges. Traditionally, float-operated water level gauges have been installed in harbours etc. Submersible, self-contained pressure gauges are located offshore. In addition to charting and navigational applications, water level information is used for coastal engineering studies, resource planning, etc.
A navigator requires information on the horizontal movement of water (currents) in addition to tidal information. The usual method of obtaining the data is to suspend several current meters, which automatically record speed and direction of currents, at specific depths on a single mooring line. The line is anchored to the sea bed and supported by a submersible buoy. Two of the methods used are illustrated in Figure 5.11. The current meter (left side of figure) is equipped with an acoustic release device which can be activated by a signal from the survey ship, allowing the float to rise to the surface. On the right side of the figure is an array of current meters set at different depths and utilizing surface buoys to assist in the recovery of the meters. Data is recorded automatically on magnetic tape and the meters can be left submerged for up to 12 months.
Figura 5.11 Techniques for measuring current speed and direction. (After Canada, Department of Fisheries and Oceans, 1979)
Other than the data and its mode of collection, the production phases of a hydrographic chart are identical to those of the topographical map, namely:
i) collection of soundings etc., by a survey vessel;
ii) collation of this information in the hydrographic operations centre of the survey vessel, where data is checked for accuracy;
iii) plotting the soundings on a field sheet of the area surveyed;
iv) transference of field sheet data to a compilation copy of the nautical chart. (In the compilation process the field sheets and other source data are photographically reduced to chart size. Once a mosaic of this source data has been prepared, cartographers select the data to be shown on the chart.);
v) digitizing of the graphic data, i.e. conversion into computer compatible form for automatic drawing by a computer controlled plotter. (The plotter produces high quality negatives for each colour shown on the chart. The negatives are passed to the printing units for platemaking and printing.);
vi) amendment of chart catalogues to indicate coverage of new areas or update of previously mapped areas. (In the latter case “Notices to Mariners” are distributed.)
The size of the area to be surveyed and the scale of the chart to be produced are carefully coordinated. The largest scale surveys are generally those for harbour charts which show more detail than general navigation charts. More general surveys and smaller scales are used for extensive offshore areas in which hazards to navigation are few.
The nautical chart and the topographic map constitute the primary forms of mapping which may be used as base maps or from which base maps can be derived. Base maps constitute the skeletal structure on which thematic information is overlaid to produce thematic maps.
From the previous section it will be appreciated that nautical charts and topographic maps differ in a number of important respects, some of which have relevance to thematic base map preparation:
i) Projection: Nautical charts generally use Mercator, whereas, topographic maps use Transverse Mercator (refer to Section 3);
ii) Symbology: Different in most respects (Figure 5.12 a-d);
iii) Coordinate system: Nautical charts have parallels of latitude and meridians of longitude, and sometimes a Loran and/or Decca lattice. Topographical maps have parallels of latitude, meridians of longitude, and grid squares (refer to Section 4);
Figure 5.12 a Nautical chart symbols. (After Canada, Department of Fisheries and Oceans, 1981)
Figure 5.12 b Nautical chart sumbols.(After Canada, Deaprtment of Fisheries and Oceans, 1981)
Figure 5.12 c Nautical chart symbols. (After Canada, Department of Fisheries and Oceans, 1981)
Figure 5.12 d Topographic map symbols. (After C.L. Blair and R.I. Simpson, 1978)
iv) Distance: In nautical charts, which generally use Mercator projections, one minute of latitude is always equivalent to one nautical mile (1852 metres or 6080 feet). In topographical maps, various other projections are used so there is no constant equivalence (refer to Section 3);
v) Bearings: Nautical charts have two or three compass roses (Figure 5.13) in contrast to a declination diagram of the three norths on topographical maps (Figure 5.14). This illustrates the relatively greater importance of the compass in a marine environment;
vi) Terminology: Variation, the angular difference between true north and magnetic north on hydrographic charts, is termed declination on topographic maps;
vii) Coastlines: Coastlines are naturally of critical importance to marine mapping. Their compilation for small-scale maps is relatively simple because they usually require so much simplification that detail is of little consequence. When compiling medium and large-scale maps, however, the major difficulties facing the cartographers and hydrographers include the following:
a) Datums: Hydrographic charts use low water reference datums (refer to Section 4.4), whereas, topographic maps use Mean Sea Level. As a result, the shape of the coast will differ, particularly in areas of high tidal amplitude;
b) Colouring: There are a number of inconsistencies when utilizing both charts and maps, e.g., marshland, definitely not navigable, is likely to be coloured as land on a chart, whereas a low-lying swamp on a topographic map is likely to be coloured blue as water;
c) Geomorphological changes : In some areas of the world, the shape of the coast changes rapidly due to erosion or deposition (refer to Section 13.6.12). These changes may be monitored by the comparison of historical and current aerial photography and satellite imagery;
d) Scale: In some conventional projections the scale varies considerably over the map, particularly in the higher latitudes, giving certain areas of the coast undue emphasis.
Figure 5.13 Compass rose
Figure 5.14 Declination diagram. (After C.L. Blair and R.I. Simpson, 1978)