LA561 Computer Applications in LA
(GIS)
Map Projection
Dr. Ragab Khalil
Department of Landscape Architecture
Faculty of Environmental Design
King AbdulAziz University
Room 103
Overview
1.
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Geographic coordinate system
What is a map projection
When should you use a projection
Spatial distortion and types of map projections
How to Choose a map projection
Map projections used commonly in KSA
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
2/31
Projection: An Introduction
• We use maps to model the earth.
• A map is a representation of the spatial organization of
the physical universe at any scale.
• A map contains a wide variety of information, both
static and dynamic.
• Maps are usually created on a flat, two-dimensional
surface.
• An important step in modeling geographic phenomena
on a map is the conversion of the 3D world into a 2D
model.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
3/31
1
Coordinate systems
• We have two types of coordinate systems
Geographic coordinate system
Projected coordinate system
An ellipsoid, datum, a projection and units
make up a coordinate system
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
4/31
Geographic Coordinates
• The Geographic Reference System consists of latitude
and longitude.
• This system treats the globe as if it were a sphere.
• The sphere is divided into equal parts called degrees.
• The standard origin is where the Greenwich Prime
Meridian meets the Equator.
• All points north of the Equator or east of the Prime
Meridian are positive.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
5/31
Geographic Coordinates
• Lines of longitude (meridians) run
north and south and measure the
number of degrees east or west of
the Prime Meridian. Values range
from -180 to +180 degrees.
• Lines of latitude (parallels) run
from east to west and measure the
number of degrees north or south
of the Equator. Values range from
the North Pole, at +90 degrees, to
the South Pole at -90 degrees.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
6/31
2
What is a map projection?
• Due to the fact that the Earth is a sphere (3D) and a
map is flat (2D), we need to convert locations from a
curved surface to a flat surface.
• The mathematical formula used to do this is called a
map projection.
• A map projection is a result of the process of
systematically transforming positions on the Earth’s
spherical surface to a flat map, while maintaining
spatial relationships.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
7/31
What is a map projection?
• A light positioned in the center
of the sphere shines
("projects") outward, casting
shadows from the lines
• Shadows are cast upon a
surface outside the sphere
• The surface is opened flat
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
8/31
When should you use a projection?
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When using measurements to make important decisions.
When measuring distance or direction.
When comparing geographic features/phenomena.
When you are analyzing spatial relationships of data.
Aligning image and feature themes.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
9/31
3
Distortion due to projection
• Map projections are not perfect! It is impossible to
convert a 3D set of features to a 2D set of features
without some errors.
• The errors caused when applying a map projection are
called distortion.
• Distortion can affect one or more of these spatial
properties:
Shape
Area
Distance
Direction
another)
Dr. Ragab Khalil
(the geographic shape of a feature)
(the amount of area of a feature)
(how far away features are from each other)
(the orientation NESW traveling from one feature to
KAAU - FED – LA561: GIS
10/31
Distortion due to projection
• Depending on the use of the map certain spatial
properties will be more important than others.
• The amount of distortion caused by a map projection
increases as you travel away from the reference origin.
• By choosing a projection centered on the location being
mapped, the effects of distortion will be minimal for the
area of interest.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
11/31
Categories
• Map projections can be categorized according
to the spatial property they preserve.
Property
Area
Shape
Category
Equal-area
Conformal
Distance
Direction
Equidistant
Azimuthal
NOTE: It is impossible to construct a map projection that
is both equal area and conformal.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
12/31
4
Types of Projections
• Conic
(Albers Equal Area, Lambert Conformal Conic) , good for EastWest land areas, parallels are concentric circles
• Cylindrical
(UTM), good for North-South land areas, parallels are straight
• Planer
(Lambert Azimuthal Equal Area), good for global views,
parallels are eccentric circles
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
13/31
How to choose a map projection
Questions to ask when choosing a map projection:
1. Is my other data already projected?
If the other data you are using is already projected then you
must project your data to the same projection as this data.
2. Which spatial properties are most important to me?
you must choose which spatial properties are most important
for what you are doing: shape, area, distance or direction.
Choose a map projection that protects these important spatial
properties.
3. What is the location and scale of my data?
Map projections have been specially designed for different
areas of the earth and also for different map scales.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
14/31
Example of differences due to
different projections
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
15/31
5
Example of differences due to
different projections
Small scale
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
16/31
Map Projections Used in KSA
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Coordinate system: Universal Transverse Mercator (UTM)
Spheroid: International [semi-major= 6378388.0 m, 1/f= 297.0
Datum: Ain Alabd 1970
Zone number: 37, 38, 39, 40 with central meridians 39, 45, 51,
57 East respectively
• Scale factor: 0.9996
• Lambert Conformal Conic projection is also used
• The first and second standard parallels are 17 and 33 N
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
17/31
Map Projections Used in KSA
• The Mercator projection, however, is conformal;
therefore, it is excellent for navigational purposes but
only preserves local shapes.
• Countries in the mid-latitudes are best represented
by a Conic projection
• The result is less overall distortion of land and water
areas.
• The Lambert Conformal Conic projection is a
commonly used Conic version.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
18/31
6
Modeling the Earth’
Earth’s shape
• Spherical models are useful for small-scale
maps (features are small) such as world
atlases since the error at that scale is not
usually noticeable or important enough.
• The ellipsoidal model is commonly used to
construct topographic maps and for other
large and medium scale maps that need to
accurately depict the land surface.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
19/31
Reference Ellipsoids
•A spheroid ( ellipsoid) is a mathematical description of
the earth.
•It is usually defined by semi-major (equatorial radius)
and flattening (the relationship between equatorial and
polar radii).
Semi-minor axis
Semi-major axis
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
20/31
Reference Ellipsoids
• Many reference
ellipsoids are in use
by different nations
and agencies.
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
21/31
7
Datums
• A datum is a set of control points whose geometric
relationships are known, either through measurement or
calculation, and is used to define a coordinate system.
• Datums are always based on ellipsoids that best
represent the geoid within the region the datum is going
to be used for.
Geoid
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
22/31
Datums
• Commonly used datums in Saudi Arabia
Ain Al’Abd 1970
World Geodetic System of 1984 (WGS84)
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
23/31
UTM
• Universal Transverse Mercator (UTM)
Every place on earth falls in a particular zone
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
24/31
8
UTM
• Universal Transverse Mercator (UTM)
Every place on earth falls in a particular zone
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
25/31
Summary
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
26/31
Projecting data frames
start with unprojected data
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
27/31
9
Projecting data frames
then apply a projection to the data frame
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
28/31
saving data sets in projected
units
• Export from a projected data frame
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
29/31
Projecting spatial data sets
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Utility for converting data sets from one system to another
• Used for going between projections
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
30/31
10
Next week
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1st Quiz
5 points
Time 15 minutes
Covers all lectures
Dr. Ragab Khalil
KAAU - FED – LA561: GIS
31/31
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