Projection

A map projection is how the round earth gets onto a flat map or screen. USGS Professional Paper 1395, "Map Projections: A Working Manual" by John P. Snyder (1987), defines it as "a systematic representation of all or part of the surface of a round body, especially the Earth, on a plane." It continues: "Since this cannot be done without distortion, the cartographer must choose the characteristic which is to be shown accurately at the expense of others, or a compromise of several characteristics."

What gets distorted

USGS says a flat map "can show one or more--but never all--of the following": true directions, true distances, true areas, and true shapes. The two main families:

• Conformal: local angles and shapes are kept. Snyder notes that "Nearly all large-scale maps of the Geological Survey and other mapping agencies throughout the world are now prepared on a conformal projection."
• Equal-area: areas are kept, while shapes, angles, and scale are distorted on most of the map.

Snyder is blunt: "No map can be both equal-area and conformal."

Projections in U.S. land work

• Transverse Mercator and UTM: the Universal Transverse Mercator grid uses the Transverse Mercator in 60 zones, reducing the scale of each zone's central meridian "to 0.9996 of true scale" to hold distortion down.
• State Plane: NOAA's National Geodetic Survey calls the State Plane Coordinate System "a system of large-scale conformal map projections originally created in the 1930s." Snyder explains that it used the Lambert Conformal Conic "for States of larger east-west extension and the Transverse Mercator for States which were longer from north to south."
• Mercator: USGS notes that "distances and areas on Mercator projection maps are grossly distorted near the map's polar regions."

Web services often use a display projection. BLM's national PLSS service and the USGS Topo basemap are both published in spatial reference 3857. See Coordinate System.

Grid is not ground

BLM's Manual of Surveying Instructions (2009) warns about State Plane grids: "The grid bearing is a true bearing only along the central meridian of a given SPCS zone, and a grid distance is not generally a true ground distance. Also, acreage on the grid surface differs from the ground surface acreage used in the PLSS" (section 2-34).

Why it matters in mining claim research

Every acreage and distance a GIS reports depends on the projection. A projection chosen for display is for looking, not measuring. Even a well-chosen State Plane figure is grid acreage, not the ground acreage the PLSS uses. Measurements of mining claims belong in a projection suited to the area, with the projection written next to the number, and compared against record acreage rather than substituted for it.