Topographic Maps and GIS

Topographic maps are maps that represent the surface of the earth. Although topographic maps are two dimensional, they indicate what the shape of the surface is like using contours (USGS 2005). Contours are simply lines on the map that join areas of equal height (i.e. height above sea level or ocean bottom depth)(USGS 2005). Important to note is that contours never cross (USGS 2005). Thus contours can be used to ascertain physical characteristics such as:

  • height – e.g. of mountains or ocean depth and
  • slope – e.g. of hills, river valleys and ocean gradients (USGS 2005).

The intensity (and shape) of slope can easily be determined by observing:

  • how the contours converge and – i.e. the closeness of the lines and the angles of convergence, and
  • how many contours converge at any point on the map.

On 1: 50 000 toposheets (referred to here as undigitalized, paper or linen topographic maps) or GIS topographic maps (digitalized computer generated or scanned maps) the contours will have set height intervals separating the contours. When dealing with terrestrial areas the heights are indicated as heights above sea level. The heights are indicated along the contours so that differences in height can be calculated by subtracting the greatest height (first point – e.g. beacon on the top of a koppie or hill) from the smallest (last point – e.g. bottom of a river bed). Therefore one can easily determine, by quick observations on a map, where a slope will be steeper as the number of contours converging increases at closer distances. Traditionally we need to construct cross profile drawings in order to visualise the lie of the land from a toposheet, however using GIS applications it is possible to tilt or even rotate the angle of view on a map so that a three dimensional perspective is achieved.

Symbols and Topographic Maps

Topographic maps are useful in that they have additional information about the physical surface by providing symbols to represent various natural or man-made features (USGS 2005). GIS maps may also provide useful information using symbols, however the symbols are often presented as optional layers that can be switched on and off. This has an added advantage in that using selected layers provides information relevant to the user. In addition the ability of simplifying the map (i.e. the user chooses to have only roads and schools represented on the map) speeds up the interpretative time.

Since there is almost no limit to the information attached to a GIS map, combinations of thematic, chloroplethic and topographic maps can be overlaid. Basically any information with a spatial component can be represented within a GIS (Knight 2005). Thus GIS can provide greater amounts of information than standard toposheets. This does not suggest that toposheets are becoming obsolete. Field navigation and interpretation of features in the field is often made more practical using toposheets, where laptop computers may be too cumbersome. In contrast GIS maps are seamless and do not require that maps be printed out for use owing to their digital format, although maps can be printed if necessary(at any scale)(Knight 2005).

Symbols are discussed here not as single units only and include attributes such as shading and colour coding. With this in mind the three most noticeable features on a topographic map include:

  • vegetation (green)
  • water (blue) and
  • densely build-up areas (gray or red) (USGS 2005).

Some more examples of what symbols can represent include features such as pans, roads, railways, hospitals, schools and boundaries. As areas represented by maps change, they are updated, sometimes requiring changes in the use of symbols (i.e. single houses represented by black dots may change to gray shading as the area becomes more densely build-up) (USGS 2005). All toposheets will have the date, place and time of publishing printed on the map (Knight 2005). An enormous advantage of GIS maps is that they can be updated immediately due to their digital format (Knight 2005). This is useful when data is continually streamed into a data bank, which can thus be updated (i.e. yearly, monthly, daily, hourly etc.).

Topographic Maps and Position, Projection and Spatial Information

Position

The position of topographic maps is indicated at the top of the 1: 50 000 map (e.g. 3317BB and 3318AA SALDAHNA). This position of Saldahna indicates precisely the position of the represented land surface in terms of degrees of latitude and longitude (Knight 2005). The 1 X 1 block below is the system that the above reference position for Saldahna is based on, however this requires some interpretation.

Firstly, it must be noted that the block is divided into quarter degree blocks (i.e. 15 per parallel and meridian)(Knight 2005). Secondly, if one takes position 3317BB as an example, this represents 33 south and 17 north, both values being situated at the top left corner of the block (Knight 2005). From this the position of each block can be determined by simply counting the quarter degree blocks downwards and then to the right.

Lastly, using the example of the Saldahna map position, the two reference points have BB and AA attached to the degree coordinates. This means that BB belongs to the big B and the small B of the left block in Fig 1 and the AA on the opposite side of the right block. Thus the Saldahna map title indicates that the map is composed of the two quarter degree squares BB and AA indicated in Fig 1.



Fig 1 A 1 X 1 block

Source: Knight (2005)

Projection

Accuracy of map projections is highly dependent on the type of projection method used to display the map. It is difficult to understand the level of distortion on a map projection at the 1: 50 000 scale unless a map of the entire earth can be looked at for that particular projection type. Furthermore, standard orientation of the earth on world maps generally show a pattern of increasing distortion as the distance of parallels increases from the equator (Knight 2005). Thus the Saldahna map example situated 33 south would infer a low level of distortion for the Gauss Conform Projection.

Scale

A map scale of 1: 50 000 means that for every 1 cm represented on the map, an equivalent of 50 000 cm is represented in reality on the earth’s surface. It is therefore possible to work out distances between points by making measurements on the map and converting to kilometres or metres etc. to get the true distances at the earth’s surface. Measurements on the map can be made using a ruler (straight lines) or dividers (skew lines) or string (curved lines).

Topographic maps usually include some sort of conversion table that can be used as a quick reference to convert measurements of says metres with feet. In addition almost all toposheets have a ruler or measurement bar that has pre-calculated ‘actual’ distances that can be applied as a quick reference tool to determine distances between points. This is usually situated at the bottom of the map. Again these measuring tools usually include a series of measurement bars with say metres, feet and miles. It must be stressed here that GIS can be used to perform the above functions with greater speed and accuracy. In addition, is would be possible to change the scale of a digital map if this was required by the user.

Spatial distortion

Care must be taken when taking any distance measurements so that two aspects of distance and space are considered. These include:

  1. distance as the crow flies (calculated from distance between points on the map) and
  2. walking distance (that must consider what effect gradient imposes on the distance measured)

Point number 2 is related to area measurement that can also be determined from toposheets, although one must take into consideration the gradient of the earth’s surface. This method is too lengthy to discuss in any great detail here, however it must be emphasised that this is a timely and arduous procedure. GIS can be far more useful here since software packages can be applied to perform these functions more speedily and with greater accuracy. Thus for obtaining spatial information from topographic maps, GIS are better suited for a range of tasks.

Finally,a GIS is better equipped to perform search functions since such functions can be made according to set parameters (Knight 2005).

References

  • Knight R. 2005. Maps, Map Projections and Reading Maps. NISL bioinformatics coursework material, University of the Western Cape, Department of Biodiversity and Conservation Biology, Private Bag X17, Belville 7535. Accessed online on 11/08/05 at 08h15 from
  • USGS (United States Geological Society). 2005.Accessed online on 10/08/05 at 08h30 from erg.usgs.gov /isb/pubs/booklets/symbols/index.html