GIS Data Types

GIS data types: the type of data determines how the computer stores, processes, and displays the results.  You must undertand those distinctions, since there will be different ways to deal with each kind of data.

Image of data Data type Open format Notes

Raster: Geotiff

Raster data uses a regular grid of points to represent the data. Since the grid is regular, the x and y coordinates do not need to be stored for each point. The corner coordinates and grid spacing define the x and y coordinates for every point in the grid, which can be calculated as needed rather than stored (computer computation may be cheaper than storage and file access). Each point just stores the value at that location, although it can point to a look up table with an explanation of the coding. Finer and finer grids lead to better resolution at the cost of increased storage, which goes up as the square of the improved resolution.

Advantages: Regular grid, fast access, efficient storage unless no data for most of the area.

Some raster data (like elevations in DEMs) can interpolate values between the grid values, while others (like land cover grids) cannot.  In you interpolate the pixels might not be obvious, but the surface would become overly smooth with no detail.

This grid has been blow up far beyond its useful range, allowing you to clearly see the individual postings.

Rasters are 2.5D grid and not true 3D.

Depiction of the grid with the values.

Vector: Shapefile

Vector data uses straight line segments to represent the data. Small enough line segments can define any desired curve. For each point both an x and a y coordinate must be stored. Each line consists of a series of points, plus some information about what the line represents. A point is a line that consists of a single coordinate pair, with an area is a line that closes on itself to enclose a region.

  • Point data might be earthquakes or volcanoes. A point might be considered to be a degenerate line with only a single pair of coordinates.
  • Line data might be plate boundaries, rivers, or hurricane tracks.
  • Area/polygon data might be countries, counties, or lakes. It is a line that closes on itself and encloses a region.

The geometry might change with scale.  At small scale, a city might be a point, but at large scale it would be an area.  Similarly, at small scale a river would be a line, but at large scale you might better depict it as an area.

This vector data set has been blow up far beyond its useful range, allowing you to clearly see theindividual line segments.

Advantages: variable storage, indexing, and the easy ability to associate attributes with the data.

Vector data is generally only 2D, but the shapefile format can have point elevations as an attribute in the database, or included in the SHP file (while this is allowed, it is seldom included in practice).

Vector data usually includes a database.

Point clouds LAS Series of x,y,z coordinates that define positions in space (true 3D).  The points are usually distributed quasi randomly.  Often there is additional data. This is a relatively new data type, with the amount of available data increasing rapidly.

The measure of scale here is the point density, given either as the number of returns per square meter, or the distance between points on the ground.

Tradeoffs

Each type of data is collected at a target scale, which balances computer storage, display speed, and collection costs versus the level of detail needed. While the computer can easily change the scale  for display, this may have severe and undesirable impacts on the data.

Type of Data Opening Data Display Control Most common Format
Grid/Raster Geotiff
Vector Shapefile
Point Could LAS files

Last revision 1/21/2018