Raster Data

Raster data stores a place as a grid. Esri's ArcGIS Pro documentation: "In its simplest form, a raster consists of a matrix of pixels (or cells) organized into rows and columns (or a grid) in which each pixel contains a value representing information, such as image reflectance or temperature." Esri lists the sources as "digital images collected by aircraft, drones, satellites, ground and water-based sensors, digital pictures, and scanned maps."

Three kinds of raster

Esri sorts rasters into:

• Continuous data: imagery and physical values that vary across the land, such as elevation and temperature. A DEM is the standard example. See DEM.
• Thematic data: classes such as land use and soils, often made by classifying imagery.
• Pictures: scanned maps, drawings, and photographs.

Cell size is the resolution

Every cell covers the same width and height. Esri: "The smaller the pixel size, the smoother or more detailed the image," and "if the pixel size is larger than the object of interest, that object may not exist in the raster dataset." Detail costs storage: "changing pixels to one-half the current size requires as much as four times the storage space."

A raster is placed on the earth by a few geographic properties stored with it, starting with a reference x,y location, "typically the upper left or lower left corner of the image." The cell values then follow in order, row by row.

Where rasters fall short

Esri lists the tradeoffs, including "Spatial inaccuracies exist due to the limits imposed by the raster dataset cell dimensions" and "Loss of geometric precision accompanies restructuring data to a regularly spaced raster-cell boundary." Lines and boundaries are usually kept as vector data for that reason. See Vector Data.

Rasters in public land data

• USGS 3DEP elevation models, from 1-meter lidar DEMs to 1 arc-second seamless coverage
• IfSAR orthorectified radar intensity images, which USGS says are "available only in Alaska"
• Basemaps served as image tiles, such as USGS Topo, which USGS calls "a tile cache base map service." See Tile Service.

Esri notes a common use: orthoimages displayed under other layers "allow map users to confirm that map layers are spatially aligned and represent real objects."

Why it matters in mining claim research

Imagery and terrain rasters show what is on the ground: roads, workings, disturbance, and drainages. Mining claim boundaries and PLSS lines belong in vector form, where a corner is a coordinate rather than the nearest cell. When a scanned plat or mining claim map is brought in as a raster, it sits only where its georeferencing puts it, so it should be checked against known corners before anything is measured from it.