# Vertical Perspective Azimuthal Projection

## Graticule

The graticule described is for a polar aspect.

Meridians: Equally spaced straight lines intersecting at the
central pole. The angles displayed are true angles between meridians.

Parallels: Unequally spaced circles centered on the central
pole. Spacing decreases away from this pole. The opposite hemisphere
cannot be shown, nor can distant parts of the closer hemisphere. The
limit of visibility depends on the observation altitude.

Poles: The central pole is a point. The other pole is not shown.

Symmetry: About any meridian.

## Features

This is a perspective projection on a plane tangent at the center
point from a finite distance. Scale is true only at the center point,
and is constant in the circumferential direction along any circle
having the center point as its center. Distortion increases rapidly
away from the center point, the only point which is distortion free.
This projection is neither conformal nor equal area.

## Remarks

This projection provides views of the globe resembling those
seen from a spacecraft in orbit. The Orthographic projection is a
limiting form with the observer at an infinite distance.

This projection requires a view altitude parameter, which specifies
the observer's altitude above the origin point. Because this parameter
is unique to this projection and because the projection does not need
any standard parallels, a special workaround is used. Rather than
add an extra map axes property just for `vperspec`

,
the `MapParallels`

property is repurposed instead.
You should assign the desired view altitude value to the `MapParallels`

property.
Provide a scalar value for length in the same units as the earth radius
or semi-major axis length used in the map axes reference ellipsoid
(`'Geoid'`

) property.

## Limitations

This projection is available only for the sphere. Data more
distant than the limit of visibility is trimmed.

## Example

landareas = shaperead('landareas.shp','UseGeoCoords',true);
axesm ('vperspec', 'Frame', 'on', 'Grid', 'on');
geoshow(landareas,'FaceColor',[1 1 .5],'EdgeColor',[.6 .6 .6]);
tissot;