Other common projections are listed below.

## Homolsine

This projection is actually a combination of two other projections: the Sinusoidal and the Mollweide. The region around the equator is mapped by the Sinusoidal projection, while the mid-latitude to polar regions are mapped with the Mollweide projection. The latitude/longitude values where the switch from one to the other is documented at 44 degrees, 44 minutes, and 12 seconds. In the case of the sinusoidal, the constant, K is always one. Thus, the switch latitude is the latitude at which the Mollweide has a K scale factor of one.

This projection is supported in the spherical form only.

## Ordinance Survey Transform 1997 of Great Britain (OSTN97)

The OSTN97 Transformation is a datum shift. Unlike most datum shifts, this one is applied to Cartesian coordinates and not to latitude/longitude values.

Due to this fact, this shift is implemented using the following approach: after performing the normal OSGB coordinate conversion using standard Transverse Mercator projection capabilities, this variation applies the shift indicated by the contents of the OSTN97.TXT file.

The coordinate system validation process will fail if this projection variation is used and the OSTN97.TXT file does not exist in the coordinate system tables directory.

The OSTN97 Transformation is essentially the Ordnance Survey of Great Britain's equivalent to NADCON or the Canadian National Transformation. However, this transformation is applied to projected Cartesian coordinates rather than geographic coordinates (which is normally the case for datum transformations). Thus, the OSGB equivalent to NADCON or NTV2 is to apply the projection to the ETRS89 geographic coordinates, and then apply a "grid file shift" similar to NADCON to the Cartesian results.

The data file required for this shift is rather large and consists of variable length records in text form. Upon first use, converts the data file into binary form for high performance access. You will likely encounter a pregnant pause when the OSTN97 transformation is used the first time. uses the modification date of the text file and binary version thereof to determine if the binary file needs to be recreated.

## Robinson

A projection commissioned by Rand McNally and Company in 1961 and created by A. H. Robinson. They wanted a new projection for its world maps with a view to minimizing area and angular distortion. It is neither conformal nor equal-area, but a compromise between the two. With this projection, more than 75% of the earth can be shown with less than 20% error from its true scaled size.

This projection is supported in the spherical form only.

## Sinusoidal

A longitude of origin is specified defining the only straight line of longitude on the map. All other lines of longitude are sinusoidal curves.

The Sinusoidal projection is also used as one of the two projections used to produce the Holmolsine projection.

This projection supports both elliptical and spherical forms.

## Van Der Grinten

Introduced and patented in 1904 by A. J. Van Der Grinten, it is only used for world maps and was the standard projection for National Geographic maps of the world until 1988.

Many school geography textbooks had world maps in them using it. The Van Der Grinten is neither equal-area nor conformal. The central meridian and the Equator are straight lines, all other meridians and parallels are arcs of circles. Polar regions are severely distorted and most maps in this projection do not show the poles.

This projection is supported in the spherical form only.