UNDERSTANDING MAPPING & SCALES SGI1014.ppt

UNDERSTANDING MAPPING & SCALES

1. SEPTEMBER 23, 2012

2. 1.Definition of Scale
2.Types of Scales
3.Difference between maps and plans
4.Understanding projections
5.Use and creation of symbols for representation of
land features

3. Definition of Scale:
Scale is a ratio of the ground length to the map or
plan length. Hence a scale of 1: 1000 indicates that 1
unit on the paper represents 1000 units on the ground.
Such a scale is called a NATURAL SCALEand
therefore can be used with any unit of measure.

4. Scales can be expressed as a:
Representative Fraction (R.F. 1 : 1000)
Statement (1cm = 100m or 1 cm to 100m)
Bar Scale

5. STATEMENT SCALE
The simplest form of map scale.
E.g. 1 cm = 1 km
Or
One centimeter on the map represents one
hundred kilometers on the earth’s surface.

6. REPRESENTATIVE FRACTION OR RATIO
Uses a fraction (or a ratio) to show the relationship
between units on a map and units on the earth’s
surface:
E.g. 1:50000 or 1/50000
1 km = 1000 m = 100,000 cm
With a scale of 1:50000
÷50,000 by 100,000 .... 50,000/100,000 = 0.5 !
* Therefore a map using the scale of 1:50,000 has a
scale of 1cm = to 0.5 km or 500m

7. BAR OR GRAPHIC SCALE
Uses a ruler that is divided into units of distance.
You would compare the distances on the map to the distances
shown on the ruler.

8. MAPS and PLANS
The difference between maps and plans is that the
maps scale is too small to allow every
feature/detail to be properly represented to scale.
Hence symbols are used to depict important
features which would otherwise be too small to be
recognised.
Plans, show all the features on the ground
correctly to scale.

9. A small scale map shows a large area with a small
amount of detail.
A large scale map shows a small area with a large
amount of detail.

10. Small Scale Maps:
1 : 20,000 –1 : 1,000,000
Large Scale Maps: 1 : 1,250 –1 : 10,000
Cadastral Plans: 1 : 500 –1 : 4,000
Site Plans: 1 : 50 –1 : 500
Detail Plans: 1 : 1 –1 : 20

11. PROJECTIONS –
Definition:-A projection is any means of representing
the lines of longitude and latitude of the globe on a
flat sheet of paper. It is however impossible to
represent the curved surface of the sphere
(earth/globe) on a plane, flat surface without some
distortion. Hence there is no perfect projection but
certain projections are correct in some respects
e.g. scale may be correct along certain lines

12. There are different types of projections but they are classified in
four main groups.
1.Cylindrical Projections -which uses a cylinder wrapped around the
globe with the point of projection is at the centre of the earth.
2.Perspective Projections –in which the earth’s surface is project
from a point to a tangent plane. If the point of projection is at the
centre of the earth the perspective projection formed is called a
gnonomic projection.

13. 3.Conical Projections –in which the earth’s surface is
projected from the centre of the earth to a cone seated on
the globe.
4.Zenithal Projections –is a special type of conical
projection, in which the apex of the cone coincides with
the pole (north pole) and the cone is flattened out to
form a plane. Also known as a gnomonic projection with
the pole being the tangent point.

14. Orthomorphism –is the property of a projection in which the
scale is the same in all directions at a particular point, so that
small areas have the correct shape. This property is necessary
for topographical maps because it would make the shape of
features on the map similar to the actual shape on the ground.
Another property of an orthomorphic projection is that a
line of constant bearing is a straight line across the map,
also known as a rhumb line.

15. Graticule–is the network of lines formed on the
projection that represents the longitude and latitude lines.
The National Grid –is a rectangular coordinate system
devised for the following purposes:
1.Plotting:-It forms the basis for plotting the complex shape of
the graticuleand the position of survey control stations.
2.Reference:-It provides a unique reference for any feature
shown on maps whereby such features can be accurately
located.

16. 3. Classification:-All the maps of the island are
fitted into the grid system in a precise manner and
the grid reference point shown on any one map is
the same on all other maps.
NB: It forms a good basis for the present day use
of GIS and LIS data based systems.

17. Grid Networks –are based on the longitude and latitude
origin of the projection. Lines are established parallel and
at right angles to the longitude/latitude of the projection’s
origin with a false origin situated to the south west of a
country. This will allow all points/features in the country
to have a positive north and east coordinate value.

18. The parameters for JAD2001
follow:
JAD2001 PARAMETERS
Lambert Conformal Conic
Projection
Datum: WGS84
False Easting: 750000m
False Northing 650000m
Latitude of 1st Parallel: 18 N
Latitude of 2nd Parallel: 18 N
Longitude of Central Meridian:
77 W
Latitude of origin of projection:
18 N

19. DISCUSS THE USE AND CREATION OF MAP
SYMBOLS.
LAND USE MAPS etc.