Monday, May 13, 2013

A look at the different map projections in GrADS

There are a number of different possible map projections you can use in GrADS, and you may be familiar with some, or all of them, or perhaps you are only familiar with the standard default lat/lon projection used.  This entry will be less of a tutorial and more of just a detailed look at the different map projections you can use in GrADS.  However, before we get started, we do need a little tutorial section, just so you know the syntax for setting the map projection.  Setting the map project is extremely simple: Use the 'set mproj' command and provide it with a map projection from the list below:

`1: latlon     ` Lat/lon projection with aspect ratio maintained (default)
`2: scaled     ` Lat/lon aspect ratio is not maintained; plot fills entire plotting area
`3: nps        ` North polar stereographic
`4: sps        ` South polar stereographic
`5: lambert    ` Lambert conformal conic projection
`6: mollweide  ` Mollweide projection
`7: orthogr    ` Orthographic projection
`8: robinson   ` Robinson projection, requires `set lon -180 180, set lat -90 90`
`9: off        ` No map is drawn; axis labels are not interpreted as lat/lon

Example:
'set mproj nps'

That's all there is to it.  Once you have set your projection, you set the lat/lon boundaries as you normally would using the default settings, although certain map projections have intrinsic limits associated with them, which we will explore here.  So, in order to make the examples shown below, we will very simply plot vorticity from the GFS on a bunch of different map projections.  So starting out with the following code we will get this basic image, on the default cylindrical map projection,

'set lev 600'
'set mpdset hires'
'd abs(absvprs)'

 Vorticity on a Defualt latlon map projection

So, now that we have that out of the way, lets look at the other map projections.

Scaled:

The scaled map projection does not differ much from the default cylindrical projection, except that it gets rid of the aspect ratio.  Basically, using this option will squish, or elongate the map to fit your specified page area.  The image below shows the transition.  To show the different more dramatically, I set the lat/lon coordinates to -40 to 40 instead of -90 to 90.

 latlon vs. scaled map projections

Personally, I don't have much use for the "scaled" map projection, but it is always an option if you need to fit stuff together.

North/South Polar Stereographic NPS/SPS:

The NPS map projection can be useful if you are looking down at the north pole.  Pretend basically that you have the northern hemisphere, and you flatten it.  This would be the north polar stereographic projection.  For the southern hemisphere, simply take the same concept and apply it.  Now, while this map projection does not set latitude boundaries, my rule of thumb has been to set the minimum(maximum, depending on your hemisphere) latitude to be the equator.  I have trouble getting information if I look at southern hemisphere values using an nps projection.  The example below shows each full hemisphere using each projection.  Latitude set from 0 to 90 and -90 to 0.

 NPS vs. SPS

Now, you don't need to have longitude set to cover the entire globe either, or the entire hemisphere.

Lambert:

The best way to think about the Lambert map projection is as if the Earth was projected onto a cone.  In fact, often time this projection is referred to as a "conic" projection as well.  Anyway, imagine the Earth is projected onto a cone with the north pole at it's point.  The Lambert projection is basically what the map would look like if the projection was unwrapped and then flattened.  The Lambert projection requires, that your latitude boundaries exist in the same hemisphere, otherwise the projection won't work.  The below example, shows the Lambert projection applied to the northern hemisphere for the longitude coordinates from -180 to 0 (western hemisphere)

 Lambert Projection

Mollweide:

The Mollweide projection is often used to show global maps, but in GrADS there are no specific requirements for the latitude/longitude boundaries.  This projection can be neat, as it sort or combines a spherical look with a cylindrical look, with the meridians converging at the pole.  This gives a neat almost 3D appearance.

 Mollweide projection

Orthographic Projection:

The orthographic projection is another neat projection that gives a 3D spherical appearance.  In GrADS there are strict requirements for the orthographic projection.  Your latitude boundaries must span -90 to 90.  Additionally, your longitude coordinates must span 180 degrees, no more, no less.  You have the option of choosing your longitude boundaries, so long as they cover 180 degrees of territory.  The result is an image similar to this one.

 Orthographic projection

Robinson Projection:

The last map projection I am going to discuss is the "Robinson" Projection (The "off" projection is not really interesting, so I'm not going to talk about it) The Robinson projection is similar to the Mollweide projection, but with the poles kind of flattened out.  This is another commonly used map projection.  In GrADS this projection requires that lat/lon boundaries cover the entire globe.  Lat=-90 to 90, and Lon=-180 to 180 (0 to 360).

 Robinson projection

So that's it for the different map projections in GrADS.  As always, each projection is useful depending on the situation, the map, the variable plotted, etc., etc.  I hope this post has given you a little more information regarding each different type of projection.  Just remember that some of them require specific lat/lon boundaries.  Aside from that, picking the right projection is just a matter of trial and error.