GIS - Distance and Buffer
distance on sloped surface
d = | x2 - x1 | / cos θ θ = slope in degrees
point distance
distance between points
fixed distance buffer
- Buffer distance is the same for all objects in a feature class
how to calculate cost distance
- Create a grid of frictions (cost grid) to define the relative cost of traversing each grid cell - Identify source cell(s) - Calculate "cost distance" as the sum of unit costs crossed to get from one place to other places - Identify least cost path
Manhattan Distance
- One way to account for barriers is to recognize that travel can be limited to transportation routes that follow cardinal directions (N, S, E, W)
interior polygon buffers
- Rather than choose a distance "outside" of the polygon, an interior buffer ("setback") can be created (ONLY in polygons) - Use a "negative" value for the buffer distance
catchment area (cost distance)
- area within a specific "cost" of source
limitations of euclidean distance
- assumes a flat earth: especially limiting over large distances - assumes no slope in terrain: especially limiting in mountainous terrain - assumes no barriers to movement: barriers can include anything that's not a road (in the case of vehicular travel) or anything on the landscape that stands in the way
euclidean distance
- based on pythagorean theorem (a²+b²=c²) - assumes: a planar earth, no differences in elevation - does not require GIS, simple mathematical calculation - also called the "cartesian" distance
variable distance buffer
- buffer distance varies based on an attribute in the feature class
line length
- distance between nodes - distance between vertices - sum of segment lengths
polygon perimeter
- distance between vertices - sum of segment lengths
Minkowski distance
- falls between euclidean distance and manhattan distance - distance of a curved arc connecting two points
distance
- fundamental geographic concept that affects relationships and interactions between things and places - sometimes we want to measure it for input to other calculations - sometimes we want to map it - essential to understanding "space"
shortest path (raster)
- greater distances are in lighter shades - straight lines emanating from source location
cost/functional distance
- implemented as a modification to calculating raster distance - user defines relative barriers to movement by mapping the "cost" of movement for each raster cell - cost can be defined by any spatial feature or attribute that limits the ability of people, animal, ideas, diseases, etc to move across the landscape
raster distance
- measured from some source cell (or cells) - distance from each cell to source calculated as euclidean distance
dissolved buffer output
- original features attributes cannot be "separated"
buffer
- region surrounding a feature (or set of features) - region defined using a set distance from a given feature (point, line, polygon, grid cell, or group of cells) - creates new spatial object defined as the areas within a specified distance from existing feature - always a polygon
Network distance
- restricts travel along existing arcs (lines) - i.e. city streets - distance is the sum of the lengths of arcs that make up the route
autonomous buffer output
- retains input features attributes
great circle distance
- shortest distance between two points on a sphere - used for points stored in spherical coordinates - required to calculate distance in spherical coordinates, especially if points are far apart
raster buffer
- usually requires two steps: calculate distances from features, reclassify - all values less than a certain distance (the buffer distance = 1, all else = 0
ways of measuring distance in arcGIS
- vector distance tool - near: locate nearest feature, return distance - generate near table: locate k nearest features, return distances, output written to a new table
