GPS
determines a location on the surface of the Earth. The
"what" is the object or objects which will be
mapped. These objects are referred to as "Features",
and are used to build a GIS. It is the power of GPS to
precisely locate these Features which adds so much to
the utility of the GIS system. On the other hand, without
Feature data, a coordinate location is of little value.
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Feature
Types
There
are three types of Feature which can be mapped: Points,
Lines and Areas. A Point Feature is a single GPS coordinate
position which is identified with a specific Object. A
Line Feature is a collection of GPS positions which are
identified with the same Object and linked together to
form a line. An Area Feature is very similar to a Line
Feature, except that the ends of the line are tied to
each other to form a closed area.
Describing
Features
As stated above, a Feature
is the object which will be mapped by the GPS system.
The ability to describe a Feature in terms of a multi-layered
database is essential for successful integration with
any GIS system. For example, it is possible to map the
location of each house on a city block and simply label
each coordinate position as a house. However, the addition
of information such as color, size, cost, occupants, etc.
will provide the ability to sort and classify the houses
by these catagories.
These catagories of descriptions
for a Feature are know as Attributes. Attributes can be
thought of as questions which are asked about the Feature.
Using the example above, the Attributes of the Feature
"house" would be "color", "size",
"cost" and "occupants".
Logically, each question asked
by the Attributes must have an answer. The answers to
the questions posed by the Attributes are called Values.
Feature
Lists
The field data entry process
can be streamlined by the use of a Feature List. The Feature
List is a database which contains a listing of the Features
which will be mapped, as well as the associated Attributes
for each Feature. In addition, the Feature List contains
a selection of appropriate Values for each Attribute.
The Feature List can be created on the CMT hand-held GPS
data collector, or on a PC. When a Feature List is used
in the field, the first step is to select the Feature
to be mapped. Once a Feature is selected, the Attributes
for that Feature are automatically listed. A Value for
each Attribute can then be selected from the displayed
list of predetermined Values.
The use of a Feature List streamlines
the data entry process and also ensures consistent data
entry among different users in the same organization.
Exporting
to a GIS System
The final step in incorporating
GPS data with a GIS system is to export the GPS and Feature
data into the GIS system. During this process, a GIS "layer"
is created for each Feature in the GPS job. For example,
the process of exporting a GPS job which contains data
for House, Road and Lot Features would create a House
layer, a Road layer and a Lot layer in the GIS system.
These layers can then be incorporated with existing GIS
data.
Once the GPS job has been exported, the full power of
the GIS system can be used to classify and evaluate the
data.
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