Procedures of grid analysis are used when detecting unknown projections. Geometrically it is based on Tissot's Indicatrix (see the section on distortions!). But there are certain aspects which can be determined by simple visual inspection.

• If meridians and parallels intersect each other at right angles, the projections is normal (and not miscellaneous or pseudo).
• If meridians are straight, intersecting each other at one distinct point and parallels are concentric circular arcs, the projection can be conic or azimuthal. If the angle between meridians are smaller than the angle between longitudes, then the projection is conic.
• If both meridians and parallels are straight parallel lines, the projection is cylindrical.
• If both meridians and parallels are circular arcs, the projection can be stereographical or miscellaneous.
• If parallels are straight parallel lines and meridians are curves, the projection can be pseudocylindrical.
• If parallels are circular arcs and meridians are curves, projection can be pseudoconic.

Choosing projections

When planning map projections the following conditions are to be considered:

• The purpose of the map (geodesy, navigation and military maps require conformal projections, while equal area projections are better for geographical and school maps)
• The scale of the map
• The size and shape of the mapped area with regard to its latitude (circular areas can be mapped on azimuthal projections, areas along parallel lines can be mapped on conic projections, areas along great circles are usually represented on cylindrical projections)

Sometimes there are more specific conditions projections should fulfill:

• straight orthodrome lines (gnomonic pr.)
• straight loxodrome lines (Mercator pr.)
• both parallels and meridians are straight (cylindrical pr.)
• parallels and meridians intersect each other at right angles (normal pr.)
• circles on the datum (sphere) remain circles on the map (conformal pr.)
• the Pole is represented as a point (eg. azimuthal pr.)
• mild distortions (eg. miscellaneous pr.)