Buffer Analysis
A GIS operation that draws polygons a set distance around points, lines, or polygons, used to find what lies within that distance.
Detailed Definition
Buffer analysis draws a zone a set distance around a feature, then uses that zone to find what falls inside it. Esri's ArcGIS Pro Buffer tool "Creates buffer polygons around input features to a specified distance." The input can be points, lines, or polygons.
Planar or geodesic
Distance can be measured two ways, and Esri's documentation explains both:
- Euclidean buffers "measure distance in a two-dimensional Cartesian plane." They suit data "in a projected coordinate system in a relatively small area (such as one UTM zone)."
- Geodesic buffers "account for the shape of the earth." Esri recommends them when features "cover multiple UTM zones, large regions, or the entire globe," or when the map projection "distorts distances to preserve other properties such as area."
The default setting, Planar, chooses for you. Projected input gets Euclidean buffers. Geographic input with a distance in linear units, such as feet or meters, gets geodesic buffers. See Projection and Coordinate System.
Options that change the result
- Negative distances: for polygons, "Using a negative buffer distance will reduce the polygons' boundaries by the distance specified."
- Dissolve: buffers can stay separate or be merged into one feature, "removing any overlap."
- Side and end types: buffering one side of a line, squared line ends, or excluding the input polygon require an Advanced license.
- Output fields: BUFF_DIST records the distance used and ORIG_FID the source feature. A dissolved output leaves both out.
Units in PLSS work
PLSS records are written in chains. BLM's Manual of Surveying Instructions (2009), table 2-1, gives "1 chain = 100 links = 66 feet (U.S. Survey Foot)." A distance taken from field notes has to be converted before it goes into a GIS.
Why it matters in mining claim research
Buffers answer questions of distance: other mining claims within a distance of a target, a withdrawal boundary close to a proposed location, or known deposits around a block of mining claims. Two cautions apply. BLM's MLRS mining claim layer is stored in geographic NAD 83 coordinates in decimal degrees, so the buffer method and units should be set on purpose, not left to defaults. And because BLM most commonly maps a mining claim to its quarter section, a buffer around that polygon measures from the quarter-section outline, not from the staked boundary. See Spatial Join.
Related Terms
GIS
A geographic information system: software and data that store, analyze, and display information tied to locations on the earth.
Geospatial Analysis
Using location to answer questions across data layers: what overlaps, what is near, what lies where, and what follows the terrain.
Overlap Analysis
Comparing the boundaries of mining claims with each other and with withdrawals and land status to find where they coincide, then checking the records to see which overlaps are real.
Vector Data
GIS data that stores features as points, lines, and polygons defined by coordinates, each linked to a row of attributes.
Spatial Join
A GIS operation that copies attributes from one layer to another based on where features sit relative to each other, not on a shared field.