Selecting A Suitable Projection/Coordinate System For Your GIS Project
GIS in Ecology
In theory, selecting the correct projection and coordinate system for
your GIS project is straight-forward. You simply set your GIS
software to the desired projection/coordinate system and off you go.
However, there are a bewilderingly large number of projections and
coordinate systems to choose from. This means that there is a
temptation to always stick to one that you are already familiar with
or that someone else uses for a similar part of the world, but that
can leave a niggling feeling that it might not be right for what you
want to be able to do in a specific project. So what can you do to
improve your knowledge of what projection/coordinate system could be
used for a specific GIS project?
The first thing you want to do is to decide if you would be better to
use a geographic coordinate system (also known as a geographic
projection – although its not actually a true projection – see
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=About_geographic_coordinate_systems
for more information), or to use a projected coordinate system ( also
known as a projection - see
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=About_projected_coordinate_systems
for moor information). The key difference between these choices is
that geographic coordinate systems use decimal degrees as their mp
units, while projected coordinate systems us real units of length as
their map units, such as metres. This means that as a general rule if
you just want to produce a map of where things are, you can get away
with using a geographic coordinate system, but if you want to measure
anything, such as the length of transect lines or the area of features
in a specific study area, you should use a projected coordinate
system.
If you opt for a projected coordinate system, the next think you need
to decide is which of the many that are available would be suitable
for you. To do this, you need to know about the properties of
different projections, and what types of thing they are suitable for.
In particular, you need to ensure that any distortion caused when
using a specific projection to represent the curved surface of the
Earth on flat two dimensional map will not affect measurements that
you may want to make in your GIS project. A useful resource for
finding out such information is
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=List_of_supported_map_projections.
If you click on the name of a specific projection, it will provide all
the information you need to tell if it is suitable for a specific
purpose or not.
For most small scale terrestrial ecology GIS projects, there will be
generally an existing local, regional or national projected coordinate
system that you can use for your GIS project. These will have been
specifically-designed to minimise distortions for the area they cover.
For example, for GIS projects covering the UK, the British National
Grid projection can be used with confidence that it will not have any
unacceptably large distortions.
For those working in the marine environment, things can be a bit more
complicated, especially if you are working far from land. In such
cases, a good general rule for small scale projects is to use a
transverse mercator (see http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Transverse_Mercator
for its properties) or Lambert azimuthal equal area projection (see
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Lambert_Azimuthal_Equal_Area
for its properties) centred near the middle of your study area.
For GIS projects covering larger areas, whether on land or at sea,
projections such as the equidistant conic (http://webhelp.esri.com/
arcgisdesktop/9.3/index.cfm?TopicName=Equidistant_Conic) are often a
good starting point.
The last thing you need to think about is whether any of the data
layers you wish to use in your GIS project need to transformed into
the projection/coordinate system you have selected for your project
before you can use them. This is because, as a general rule, it is
usually best to ensure that all your data layers are in the same
projection/coordinate system as your GIS project. This is because it
will prevent you from accidentally basing any calculations,
measurements or comparisons on the wrong one.
Transforming data layers between projection/coordinate systems is
generally quite straight forward if both are based on the same datum.
It is more tricky if your data layer and your GIS project are based on
different datums. This is because you will need to select a specific
geographic transformation to use (see
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Geographic_transformation_methods
for more information on transformations). The transformations usually
do not have particularly informative names. For example, when
transforming data layers from the WGS 1984 datum (as is generally used
for data collected with GPS receivers) to the OSGB1936 datum (used by
the British National Grid projection/coordinate system) in ArcGIS,,
you are provided with six possible options labelled as
OSBG_1936_To_WGS_1984_1, ..._2, ..._3, ..._4, ..._5 and ..._Petroleum.
Each of these is most suitable for different parts of the UK and its
surrounding seas.
If you need to find out which transformation between two datums should
be used for a GIS project covering a specific area, you can visit this
site http://support.esri.com/en/knowledgebase/techarticles/detail/21327,
and click on the software version that you are using. This will take
you to a list of all the transformations you can do in a specific
version of ArcGIS or ArcView and what parts of the world it is
suitable for. For example, here you can find out that when working
with ArcGIS 9.3, if your GIS project is in the Shetland Isles to the
north of mainland Scotland, and you want to transform data from a
projection/coordinate system based on the WGS_1984 datum to one based
on the OSGB 1936 datums, you should use the OSBG_1936_To_WGS_1984_4
geographic transformation.