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
http://planet.botany.uwc.ac.za/NISL/GIS/Power_Point/Chap1_maps.ppt.
 USGS (United States Geological Society). 2005. Accessed online on
10/08/05 at 08h30 from http:// erg.usgs.gov
/isb/pubs/booklets/symbols/index.html