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NORTHERN RIVERS BUSHWALKERS CLUB Inc
NAVIGATION TRAINING DAY
29 August 2010
A.
Topographical Maps
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The Earth is a geoid. A map has to be represented on a flat surface which requires a
system and is very complex.
Australia is represented on topographical maps which are identified by a name and
number, eg, Huonbrook 9540-1-N. A catalogue is available from Central Mapping
Authority (CMA) for NSW maps. Each individual map indicates adjacent maps.
Forestry map very handy (if available). Sunmap maps are used in Queensland.
Different map datums are used, reflecting advances in global and space-age technology.
The relevant datum for each map is shown on the map. The most relevant map datums
are: Australian Geodetic Datum 1966 (AGD 1966); World Geodetic System 1972
(WGS72); World Geodetic System 1984 (WGS84); Geocentric Datum of Australia
1994 (GDA94). There are small differences between the datums - conversions are
shown on maps – refer to bottom LH corner of Huonbrook GDA map (top RH of AGD66
map). A GPS should be set for the correct map datum.
Map scales: 1:25,000 generally for urban/semi-urban areas, 1:50,000 for rural areas.
Some general observations for a typical 1:25,000 map are:
4 cm grids (actual measurement) = 1000 metres (1 km)
Grids are numbered horizontally (east – west) and vertically (north – south).
Latitude and longitude are shown at each corner of the map (for Huonbrook map - 15
minute range in longitude, 7 minute 30 second range in latitude).
Grids are not parallel to the edges of the map.
Topographical relief is illustrated with contours (more detail later).
The legend showing map features is self-explanatory.
Any point on a map can be located by a grid reference. The horizontal grid is given first,
followed by the vertical grid, eg, for Minyon Falls – 381 348. 38 and 34 are the primary
horizontal and vertical 1 km grids respectively and 1 and 8 are estimated positions in the
grids on a scale of 10.
The map grid location can be defined within a broader more precise system, which is
used for GPS referencing - refer to the explanation on the Huonbrook map. For the
Huonbrook map, the Grid Zone Reference is 56J and the 100,000 m Square Identification
(NP) is 05/68 (shown at the bottom LH corner of the map). The position in the grid can
be given to the nearest metre. Thus, a GPS reading for Minyon Falls is: 56J 0538105 UTM 6834813, where 105 and 813 are the number of metres after the respective 1 km
grid and UTM/UPS is the Australian position format for which the GPS has to be set.
Meaning of grid north (GN), true north (TN) and magnetic north (MN) - refer to diagram
on the map.
Contours. Refer to supplementary notes (extracts from Bushpeople’s Guide to
Bushwalking in South-East Queensland).
A contour is a line which joins points of equal elevation above sea level.
The use of contours is one of several methods of showing relief on a map. Learn to
recognise: ridges, valleys, hills, knolls, saddles, steepness of slope, direction of slope, etc.
The contour interval is usually shown on the map under the scale diagram.
NSW maps (CMA) use a standard 10 metre interval. Queensland maps (Sunmap)
sometimes use a 5 metre interval, but only show the 25 metre contours in steep terrain,
which can be very confusing.
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B.
Use of a Compass
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Refer to supplementary notes
The compass incorporates a magnetic needle that automatically aligns with magnetic
north (MN).
The compass includes a simple protractor for measuring the angle between a basic
reference line (not necessarily referenced to magnetic north) and the direction of travel
arrow.
The basic reference line is the orienting arrow on the circular movable dial of the
compass. A number of parallel lines are also marked.
Some find reading the protractor confusing because the basic reference line is on the
movable part of the compass and is not aligned first.
The direction of travel arrow is marked on the rectangular base. Although it is not the
basic reference line, it is normally the direction that is aligned first.
The angle is always read at the direction of travel arrow which aligns with the index
line (not always shown).
The basic reference line (or orienting arrow) can be aligned with either magnetic north
(MN) or grid north (GN)
A magnetic bearing (or direction of travel referenced to magnetic north) is measured by
aligning the basic reference line (or orienting arrow) with the magnetic needle.
A map (or grid) bearing (or direction of travel referenced to grid north) is measured by
aligning the basic reference line (or orienting arrow) with the map north grids.
The grid/magnetic angle changes with time (refer to map). For practical purposes, in the
Northern Rivers walking area, the grid/magnetic angle can be assumed to be 12 degrees
to the right of grid north. Thus, the magnetic bearing is always less than the grid bearing.
The only relevant directions for navigating purposes are grid north (GN) and magnetic
north (MN). Forget about true north (TN) and grid convergence.
For easy comparison of field observations with map details, the map can be oriented
using the compass. In the Northern Rivers area, the compass is laid on the map and the
map is rotated until the magnetic needle is at an angle of 12 degrees to the right of grid
north.
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C.
Navigation
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Refer to supplementary notes (extracts from Bushpeople’s Guide to Bushwalking in
South-East Queensland).
Important considerations are:
Hone your observation skills and your ability to relate field observations to the map.
Keep track of progress, marking off landmarks such as ridges, creeks, knolls, etc.
Generally plot routes to follow ridges and water courses.
Be careful travelling down ridges and up water courses – continually check bearings.
In particular, be precise in setting a ridge bearing off the top of a mountain or knoll etc.
Be aware that maps are not always accurate (particularly in heavily wooded areas).
Know your walking speeds and pace length.
Keep track of time.
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