Arba Minch University, Institute of Technology, Chapter 2

Department of Water Supply & Environmental

Engineering

GIS Concepts
Arba Minch university
Institute of Technology

1. Map Scale
Course: GIS and Remote sensing 2. Coordinate systems
3. Projections
Application

Target groups: G4WSEE Sec D, E, F & G

2

1. Map scale Talking about map scale

• Ratio of distance on the map to distance • A large denominator 1
on the ground gives a small fraction --------

a small scale map. 50,000,000
• Dimensionless: cm, or inches, or mm, or3 It shows a large area.
1
--------
• A small denominator 500,000
gives a larger fraction

a large scale map.
1
It shows a small area. --------
1 cm on map = 100,000 cm on ground 5,000
2-4
3 4

By Wondwossen B.(MSc) e-mail:

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Arba Minch University, Institute of Technology, Chapter 2
Department of Water Supply & Environmental
Engineering

Generalization and Scale Estimating precision from scale

Polygons at one scale
may be points or lines Intermediate
• A 1-pt line is about 0.0025 cm
scale map
at a different scale • Map scale / 1000 gives the approximate
precision in feet for a 1-pt thick line
1:5,000 5 ft
1:24,000 24 ft
1:100,000 100 ft
1: 1 million 1000 ft
Large scale map
Larger symbols have lower precision...
Small scale map 5 6

Source scale and display scale Scale and resolution

• Most GIS data have an intrinsic scale– • Resolution is the sampling distance of the
“inherited” from the source stored x-y values.
A larger scale map generally
• Display scale varies! Display scale approximately 1:500,000 has a finer sampling distance
and better spatial resolution.
It can represent features with
better precision.
Too fine a resolution
wastes storage space
and slows drawing—
stores more points
than needed at a
1:5M scale source 1:25M scale source
particular display
1:50,000 USGS Ethiopian Map (source scale) 2-7 scale
7 8

By Wondwossen B.(MSc) e-mail:

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Arba Minch University, Institute of Technology, Chapter 2
Department of Water Supply & Environmental
Engineering

2. Coordinate Systems Geographic Coordinate System

A coordinate system is a
Geographic vs. Projected reference system used to
represent the locations of
• Geographic Coordinate Systems geographic features,
(GCS)
• Location measured from curved surface
imagery, and observations
of the earth such as GPS locations
• uses a three-dimensional spherical within a common geographic
surface to define locations on the earth.
• A point is referenced by its longitude and
framework.
latitude values. •Parallels - E to W– 0° at the
– Degrees-minutes-seconds (DMS) Equator (0 °-90 °)
– Decimal degrees (DD) or radians (rad)
•Meridians – N to S – 0° at
• Projected Coordinate Systems (PCS) the Prime Meridian (0 °-180 °)
• Flat surface
• Units can be in meters, feet, inches
• Distortions will occur, except for very fine
scale maps
•Latitude and longitude are angular measurements made from
• Visualize a light shining through the Earth the center of the earth to a point on the surface of the earth
onto a surface

Spheroids
• The Earth is not a perfect spheroid. Different spheroids Map Projection:
are used in different parts of the world to create the best
possible model of the Earth’s curvature in each location.
• A map projection is a
mathematical formula for
representing the curved
surface of the earth on a
flat map.
– wide variety of projections
possible
– each projection will create a
different type of distortion

By Wondwossen B.(MSc) e-mail:

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Arba Minch University, Institute of Technology, Chapter 2
Department of Water Supply & Environmental
Engineering

Distortions Distortions are inherent in maps

The Earth is round, a map is flat
Map Projections Parameters
Standard Line – line of tangency - where the projection
surface meets the globe
•No distortion at this point – increase
distortion moving away
•Standard Parallel
•Standard Meridian
•More than one standard line possible

•Central Line
• distance •Defines the center of the map projection
• area •Central parallel - latitude of origin
• shape •Central meridian
• direction •Can be different from standard line
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Map projection classifications

Map Projections Types
According to what spatial attribute they preserve.

Equal area projections preserve area

Conformal projections preserve shape. Mercator projections
Equidistant projections preserve distances,.
Planar Projection Conical Projection Cylindrical Projection
Azimuthal projections preserve direction from one point to all
other points.
Other projections minimize overall distortion but don't
preserve any of the four spatial properties of area, shape,
distance, and direction. 14
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By Wondwossen B.(MSc) e-mail:

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Arba Minch University, Institute of Technology, Chapter 2
Department of Water Supply & Environmental
Engineering

Cylindrical Aspects
Planar Aspects

Conic (tangent)

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Commonly Used Map Projections Universal Transverse Mercator (UTM)

• 60 zones
– 6o wide
• Adidan – Defined by central meridian (example: 120° W)

• Universal Transverse Mercator (UTM)

• Preserves direction and small shapes Secant!
84 N°
(conformal projection).
• Extent is from 84°N to 80 °S.
80 S°
• UTM coordinates are easily recognized by 6
digit for the x, and 7 digit for the y

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Arba Minch University, Institute of Technology, Chapter 2
Department of Water Supply & Environmental
Engineering

Universal Transverse Mercator (UTM)

UTM
• Often found in military applications, and in
datasets with global or national coverage
• UTM system is secant with lines of scale 1
located on both sides of the central meridian,
the projection is conformal, so small features
appear with the correct shape and scale is
the same in all directions. (Scale is 0.9996 at
the central meridian and at most 1.0004 at
the edges of the zones
• Because there are effectively 60 different
projections, maps will not fit together across a
zone boundary.

Universal Transverse Mercator (UTM)

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