51
6
Since there are all of these datums and map projections out there, it’s important
to know which one you’re using when making measurements. Say, for instance,
you have two sets of geospatial data that you need to bring together on one
map of your town for analysis (that is, a street map and building footprints).
Both of these pieces of data are in a different projection and coordinate system, and both use a different datum—the street map is in State Plane NAD27
feet and the buildings are in UTM NAD83 meters. When you put these things
together, nothing is going to match up—the streets and buildings are going to
be in different places in relation to one another since each one uses a different
basis for its coordinates.
It is important to note when you’re dealing with geospatial data that it
can be constructed using a variety of different datums, coordinate systems,
and projections. With all of the different variations involved with these items,
it’s best to try and work with all geospatial data on the same basis (that is, use
the same datum, coordinate system, and projection). For instance, if you have
multiple data layers in different map projections and you try to overlay them,
then those pieces of data won’t properly align with each other (Figure 3.1 on
page 52).
How Can You Align Different Geospatial
Datasets Together?
Back in Chapter 2, we discussed the concept of having to transform data measured from different datums to a common datum so that everything will align
correctly. As such, it’s best to reproject your data (or transform all of the
datasets to match one, or transform them all to a common datum, coordinate
Getting Your Data to Match
the Map
Reprojecting, Georeferencing, Control Points,
and Transformations
3
reproject changing
a dataset from one
map projection (or
measurement system)
to another.
6
Since there are all of these datums and map projections out there, it’s important
to know which one you’re using when making measurements. Say, for instance,
you have two sets of geospatial data that you need to bring together on one
map of your town for analysis (that is, a street map and building footprints).
Both of these pieces of data are in a different projection and coordinate system, and both use a different datum—the street map is in State Plane NAD27
feet and the buildings are in UTM NAD83 meters. When you put these things
together, nothing is going to match up—the streets and buildings are going to
be in different places in relation to one another since each one uses a different
basis for its coordinates.
It is important to note when you’re dealing with geospatial data that it
can be constructed using a variety of different datums, coordinate systems,
and projections. With all of the different variations involved with these items,
it’s best to try and work with all geospatial data on the same basis (that is, use
the same datum, coordinate system, and projection). For instance, if you have
multiple data layers in different map projections and you try to overlay them,
then those pieces of data won’t properly align with each other (Figure 3.1 on
page 52).
How Can You Align Different Geospatial
Datasets Together?
Back in Chapter 2, we discussed the concept of having to transform data measured from different datums to a common datum so that everything will align
correctly. As such, it’s best to reproject your data (or transform all of the
datasets to match one, or transform them all to a common datum, coordinate
Getting Your Data to Match
the Map
Reprojecting, Georeferencing, Control Points,
and Transformations
3
reproject changing
a dataset from one
map projection (or
measurement system)
to another.