Is there documentation on how to set the view altitude for the 'vperspec' projection using the Mapping Toolbox Release 2008a (R2008a)?

I am using the Mapping Toolbox to plot an approximate view of the earth from space using the Vertical Perspective Azimuthal (vperspec) Projection and would like to adjust the altitude of the view.

I can use the following to set the latitude and longitude of the view.

set(gca,'origin',[latitude longitude orientation])

However, there does not appear to be any control to set the altitude of the view. Is there documentation on how to do this?

Answer 1

[MathWorks Support Team]

This enhancement has been incorporated in documentation of Release 2008b (R2008b). For previous product releases, read below for any possible workarounds:

The VPERSPEC projection is the only projection which needs a view altitude parameter. Therefore, rather than having a dedicated map axes property, the 'MapParallels' property (which is not needed otherwise, since VPERSPEC is azimuthal and thus has no standard parallels) is re-purposed just for this projection. Hence, setting this property to a scalar value controls the observer's altitude above the origin point. This value should represent a length, and needs to be in the same units as the earth radius or semi-major axis length used in the map axes reference ellipsoid ('Geoid') property.

This modified behavior of the 'MapParallels' property is documented in the file help and reference page for VPERSPEC, as follows:

        "The standard parallel can be interpreted physically as the altitude of the

        observer above the surface in the same distance units as the geoid."

The altitude of a VPERSPEC projection can be set as follows:

1. Initialize a vertical perspective azimuthal projection with the observer at a height of one earth radius:

Re = almanac('earth','radius','km')

axesm('vperspec','Geoid',Re,'MapParallels',Re,'Grid','on','Frame','on')

2. Now, increase the view altitude to 42,000 km:

view_altitude = 42000;

setm(gca, 'MapParallels', view_altitude)

For more information on the 'MapParallels' property of a Map Axes please visit the documentation page given below:

 <http://www.mathworks.com/access/helpdesk/help/toolbox/map/axesm.html#f12-65586>

If you have installed the help documentation, you may access the same page locally by typing the following at the MATLAB prompt:

web([docroot,'/toolbox/map/axesm.html#f12-65586'])