38
Chapter 2 Where in the Geospatial World Are You?
world onto the surfaces of the walls. To make a map projection, you’d have
to wrap a piece of paper around the globe and “project” the lines onto that
paper. Then, when you unroll the paper, you’d have the globe translated onto
the map.
The real process is a much more complicated mathematical translation
of coordinates, but the basic concept is similar. A map projection is created
through the use of a developable surface, or a flat area onto which the coordinates from Earth can be placed. In this way, the locations on Earth’s surface get translated onto the flat surface. The three main developable surfaces
(Figure 2.5) that are used in this fashion are a cylinder (which creates a
cylindrical projection), a cone (which creates a conical projection), and a flat
plane (which creates an azimuthal surface). The place where Earth “touches”
the surface is called the point of tangency, which is where the map’s distortion is minimized.
Like datums, there are numerous map projections available for use, each
having its own different uses and built-in distortions (see Hands-on Application 2.2: Examining the Effects of Different Map Projections for more information). Two of the more common projections that you may encounter with geospatial data are Lambert Conformal Conic and Transverse Mercator.
FIGURE 2.5 The three
types of developable
surfaces in map
projections.
Different kinds of map projections will alter the map
in various ways, depending on which parts are being
distorted and which parts are being maintained. To
interactively examine the effects of different projections on a map, open your Web browser and go
to http://projections.mgis.psu.edu. The Website
enables you to select different projections and
view how the map looks with each of them. Switch
the projection to view the world map as Mercator,
Lambert Conformal Conic, and Transverse Mercator,
and then select the option to redraw the map. How
does each of these map projections alter the map
as drawn? Try some of the other options and examine what types of distortions and changes enter the
map with each projection.
Hands-on Application 2.2
Examining the Effects of Different Map Projections
Cylindrical
Conical
Azimuthal
Chapter 2 Where in the Geospatial World Are You?
world onto the surfaces of the walls. To make a map projection, you’d have
to wrap a piece of paper around the globe and “project” the lines onto that
paper. Then, when you unroll the paper, you’d have the globe translated onto
the map.
The real process is a much more complicated mathematical translation
of coordinates, but the basic concept is similar. A map projection is created
through the use of a developable surface, or a flat area onto which the coordinates from Earth can be placed. In this way, the locations on Earth’s surface get translated onto the flat surface. The three main developable surfaces
(Figure 2.5) that are used in this fashion are a cylinder (which creates a
cylindrical projection), a cone (which creates a conical projection), and a flat
plane (which creates an azimuthal surface). The place where Earth “touches”
the surface is called the point of tangency, which is where the map’s distortion is minimized.
Like datums, there are numerous map projections available for use, each
having its own different uses and built-in distortions (see Hands-on Application 2.2: Examining the Effects of Different Map Projections for more information). Two of the more common projections that you may encounter with geospatial data are Lambert Conformal Conic and Transverse Mercator.
FIGURE 2.5 The three
types of developable
surfaces in map
projections.
Different kinds of map projections will alter the map
in various ways, depending on which parts are being
distorted and which parts are being maintained. To
interactively examine the effects of different projections on a map, open your Web browser and go
to http://projections.mgis.psu.edu. The Website
enables you to select different projections and
view how the map looks with each of them. Switch
the projection to view the world map as Mercator,
Lambert Conformal Conic, and Transverse Mercator,
and then select the option to redraw the map. How
does each of these map projections alter the map
as drawn? Try some of the other options and examine what types of distortions and changes enter the
map with each projection.
Hands-on Application 2.2
Examining the Effects of Different Map Projections
Cylindrical
Conical
Azimuthal