DEFINITION OF
RELIEF
Relief
is a general term applied to the shape of the ground in a vertical plane. The
representation of relief of the ground, above or below a datum which is
normally sea level.
On
some plane maps no relief is shown, but on all topographic mapping and almost
all maps required for military purpose some representation of relief is
necessary, though the extent to which it
is shown and the accuracy required will vary appreciably according to the scale
and purpose of the map.
ELEMENT S IN REPRESENTATION OF
RELIEF
They are two distinct elements in
the representation of relief.
There are;
·
Representation of height.
·
Representation of shape.
Representation
of height is a factual matter in which the variations will arise from the type,
density and accuracy of the information provided.
On the other
hand representation of shape may be largely artistic and the methods will vary
on different maps.
UNITS OF
VERTICAL MEASUREMENT:
The standard
unit of vertical measurement is the meter. On charts used by air forces however the feet is still unit of height.
The unit of height of used on a
particular map is stated prominently in the margin and must be checked before
use.
Height without reference to
shape is shown by fixing the height above mean sea level at selected points.
These points in descending order of accuracy may be:
v
LEVELLED HEIGHTS ( BENCH MARKS)
These are the most precise height and normally appear
only on scale of 1:25000 or larger. They are usually indicated by a symbol and
the height expressed to one or more decimal place. A bench mark is usually a
permanent mark cut on a stone built into a wall or on the side of a
triangulation pillar, the height given is the height of the mark and not the
level of the ground.
v
TRIGONOMETRICAL HEIGHTS
Trigonometrical
stations and survey control points similar accuracy are usually shown on
maps when they are defined on the ground by a pillar or other recognizable
mark.
They are usually indicated by a small triangle with
the height expressed to the nearest unit.
-
1st
order
-
2nd
order
-
3rd order
v
SPOT HEIGHTS
These are less accurate heights and normally without
any definite mark on the ground. They are selected to indicate the height of
the ground at ruling points such as tops of hills or slopes, bottoms of
valleys, ridges points, and adders, to
supplement the information provided by the contour. They are shown by a dot
with the height. The accuracy will vary , but should be as accurate as the
contours.
v
CONTOURS:
A contour is a line on the map joining points of equal
height, and is the standard method of showing relief topographic maps.
Contouring
combines accurate indication of height with a good indication of shape ,
especially when used in conjunction with spot height.
Contours are shown at regular interval ( difference in
height between successive contours), which varies according to the scale of the
map and to the type of country mapped. The contour interval is always stated
prominently in the lower margin of the map near the graphic scale. On a
1/50,000 map with average relief the contour interval may be 10 or 20 meters (
50 feet), at 1/250,000 scale is probably 50 meters ( 200 feet).
Contours are normally drawn as continuous lines
usually in brown or similar colour. Every fourth or fifth contour depending on
the vertical interval is called an Index contour and is shown by a thicker
line. This helps in reading and counting the contour to determine a height ,
contour values are placed in break made in the contour lines.
They
are placed so that they are read way up when looking up the slope. Auxiliary
contours at an intermediate vertical interval may be shown to supplement the
standard contours in flat ground, when a small size within the standard
vertical interval might be a significant feature.
Auxiliary contours are usually broken to distinguish
them from standard contours, their values are shown.
v
FORM LINES:
Form lines are approximate contours sketched to show
the general shape of the ground rather than its height. They are used when it
has not been possible to obtain accurate contours. They are usually shown by
broken lines, but are not given height values. They are likely to be found only
in poorly mapped areas.
v
HACHURES:
Hachures show
the relief by means of short disconnected lines down the slope in the direction
of water flow. The lines are short and close together on the steeper slopes,
and longer and more spaced out on the greater slopes.
This is an artistic method which can give a good shape
but no definite height information, it is used in many earlier maps but is now
seldom used, except in cuttings, embankments and steep slopes : when used for
these purpose they are usually shown in black.
v
LAYERING ( ALTITUDE TINTS):
A layer is a uniform tint applied on the map to all ground
between defined limits of height above or below a datum. By using different
tints for different layers or depth over an area. Layers are normally used in
conjunction with contours to assist the user gaining quick appreciation of
relief.
v
HILL SHADING
Hill shading is a commonly used technique to indicate
shapes, either alone or in conjunction with contours and or layers. It does not
itself relate any positive value of height. Basically, hill shading consists of
side of a hill lighting up the sunny
side to provide contrast, the darker the shading the steeper the slope on the
shadow side. The light is assumed to come from the NW corner of the map.
v
BATHYMETRIC RELIEF
Bathymetric relief ,
i.e. showing of depths below water of sea level when required on land maps is
shown in a similar way to ground relief, viz by depth values and contours
except that they are normally in blue. Their values are usually related to the
mean sea level, but in inland water they may be related to the surface
level of the water, the datum should be
stated on the map.
Hello, my name is Godwin obilor,Graduate of Surveying and Geoinformatics Engineering at federal university of technology owerri.