FS Module 2: Mapping Processes & Methods

What is a conformal map projection?

A projection that preserves local angles and shapes (but not areas). Examples: Transverse Mercator and Lambert Conformal Conic. These are used for State Plane Coordinate Systems because surveyors need accurate angular relationships.

What is the elevation factor?

The ratio that corrects for the difference between distances measured at ground elevation and distances at the ellipsoid. Elevation factor = R / (R + h), where R is the mean radius of the earth (~6,371,000 m) and h is the height above the ellipsoid. Distances at higher elevations are longer than their ellipsoidal equivalents.

What is parallax in photogrammetry?

The apparent displacement of an object caused by a change in the point of observation. In stereo aerial photos, parallax differences between overlapping images allow calculation of elevations. Greater parallax = higher elevation. Stereoscopic viewing requires 60% forward overlap.

What map scale means "1 inch = 100 feet"?

1:1,200 (since 100 ft = 1,200 inches). Common survey map scales: 1" = 50' (1:600), 1" = 100' (1:1,200), 1" = 200' (1:2,400). To convert: multiply feet by 12 to get the denominator.

What two projection types are used for the State Plane Coordinate System (SPCS)?

Lambert Conformal Conic - used for zones that are wider east-west (e.g., Tennessee, Virginia). Transverse Mercator - used for zones that are longer north-south (e.g., New Jersey, New Hampshire). Alaska zone 1 uses Oblique Mercator.

What is the UTM coordinate system?

Universal Transverse Mercator - divides the earth into 60 zones, each 6 degrees wide (from 80S to 84N latitude). Each zone uses a Transverse Mercator projection with a central meridian scale factor of 0.9996, a false easting of 500,000 m, and a false northing of 0 m (N hemisphere) or 10,000,000 m (S hemisphere).

What is a grid-to-ground scale factor?

The combined factor used to convert between grid (map projection) distances and ground distances. It combines the grid scale factor (from the projection) and the elevation factor (from height above the ellipsoid). Combined factor = grid scale factor x elevation factor. Ground distance = grid distance / combined factor.

What is photogrammetric scale and how is it calculated?

Photo scale = focal length / flying height above terrain, or S = f / (H - h), where f = camera focal length, H = flying height above datum, h = terrain elevation. Alternatively, scale = photo distance / ground distance.

What is the difference between raster and vector data in GIS?

Raster: grid of cells/pixels, each with a value (elevation, land use). Good for continuous data (DEMs, satellite imagery). Vector: points, lines, and polygons with attributes. Good for discrete features (parcels, roads, boundaries). Vector is more precise for surveying applications.

What is georeferencing?

The process of assigning real-world coordinates to each pixel/point of a raster image or dataset. It typically involves identifying control points with known coordinates on the image and applying a transformation (affine, polynomial, rubber-sheeting) to align the image to a coordinate system.

What is relief displacement in aerial photography?

The radial shift of an object's image position away from the photo center caused by its elevation above or below the datum. Objects above the datum are displaced outward from the photo center; objects below are displaced inward. Formula: d = (r x h) / H, where r = radial distance from photo center, h = object height, H = flying height.

What are contour lines and what key properties do they have?

Lines connecting points of equal elevation. Key properties: 1) Contour lines never cross (except overhangs) 2) Closely spaced = steep slope; widely spaced = gentle slope 3) Contour lines close upon themselves 4) Contour lines are perpendicular to the direction of maximum slope 5) Every nth contour is an index contour (heavier line with labeled elevation)

What is contour interval and how is it selected?

The vertical distance between adjacent contour lines. Selection depends on: terrain relief (flat terrain = smaller interval), map scale, and purpose. Common intervals: 1-2 ft for flat areas, 5-10 ft for moderate terrain, 20-50 ft for mountainous areas. Rule of thumb: CI should not create contour lines closer than 1/50 inch at map scale.

What is LiDAR and how does it work?

Light Detection and Ranging - an active remote sensing technology that measures distances by emitting laser pulses and measuring return times. Airborne LiDAR collects millions of points per second, producing dense point clouds used for DEMs, building models, and vegetation mapping. Accuracy: typically 5-15 cm vertical.

What is an orthophoto?

An aerial photograph that has been geometrically corrected (orthorectified) to remove distortions caused by camera tilt and terrain relief. Unlike raw aerial photos, orthophotos have uniform scale and can be used as maps for direct distance and area measurements.