Computing

Tips & Tricks

Reference systems & projections

Display all available PROJ.4 projections

PROJ.4

• Use proj tool directly, at the commandline

  $ proj -lp # projections $ proj -le # ellipsoids $ proj -ld # datums</code>

  • Use [[R]] ([[http://cran.r-project.org/web/packages/rgdal/index.html|rgdal]] package) to do the same thing <code> # R code: library(rgdal) projInfo(“proj”) # projections projInfo(“ellps”) # ellipsoids projInfo(“datum”) # datums</code>

  ### Search for (or by) EPSG codes The EPSG code can be a convenient way to specify a projection in [[quantum gis | QGIS]], ogr2ogr, proj etc. The saves the hassle of inputting the full PROJ.4 definition string, which can be an error prone step. However, be aware that not all projections are associated with an EPSG code.

  • Use the web: [[http://spatialreference.org

    Online spatial reference search]]

  • Use [[R]] ([[http://cran.r-project.org/web/packages/rgdal/index.html|rgdal]] package) <code> # R code: library(rgdal) EPSG <- make_EPSG() # As an example, search for Hawaii state projections EPSG[[grep(“hawaii”,|EPSG$note, ignore.case=TRUE), 1:2]] # Get PROJ.4 information for a particular EPSG code subset(EPSG, code==2784)</code>

  ### Other projection resources

  • [[http://trac.osgeo.org/proj/wiki/GenParms

    PROJ.4 wiki page]] documenting the general PROJ.4 parameters available for defining (most) coordinate systems.

  • [[http://www.flexprojector.com

    Flex Projector]] is a free, cross-platform Java application that provides a graphical interface for manipulating map projections. Designed primarily to enable users to create arbitrary new projections for custom map visualizations, this tool can be quite educational for those who simply want to get a sense of how and why projections differ.