Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 246)

User’s Manual – GPS Mode
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• Area
A map projection is equi-areal when it correctly plots areas over the
entire map. That is, all mapped areas have the same proportional
relationship to the areas on the Earth that they represent.
Common Conformal Map Projections in Surveying
• Transverse Mercator
The Transverse Mercator (TM) projection results from projecting the
ellipsoid onto a cylinder tangent to a central meridian. Scale
distortion is maximum from east to west and minimum from north to
south, so the TM projection is often used to portray areas with large
north-south extent. Distortion of scale, distance, direction and area
increase away from the central meridian.
Many national grid systems are based on the TM projection. The
Universal Transverse Mercator grid system divides the world into 60
6-degree zones. About half of the states in the U.S. use a TM
projection for their State Plane Coordinate Systems. The British
National Grid (BNG) is a TM projection with origin at 49 degrees
north latitude and 2 degrees west longitude.
• Oblique Mercator
The Oblique Mercator projection is similar to the Transverse
Mercator projection; the ellipsoid is projected onto a cylinder.
However, instead of the cylinder tangent to the ellipsoid along a
meridian, it is tangent to the ellipsoid along any great circle other
than the Equator or a meridian. This makes the Oblique Mercator
projection appropriate for regions centered along lines, which are
neither meridians nor parallels.
The Oblique Mercator projection is used for Alaska State Plane zone
1, which covers the panhandle.
• Lambert Conformal Conic
The Lambert Conformal Conic projection results from projecting a
sphere onto a cone tangent at two (or one) parallels of longitude.
Scale distortion is maximum from north to south and minimum from
east to west, so the Lambert projection is used to map areas of large
east-west extent. Distortion of scale, distance, direction and area
increase as you move away from the standard parallels.
Lambert projections are used for about half of the State Plane
Coordinate System zones in the USA.