Reviewers:
Overview
About
This lesson covers formats that CRS information may be in including proj4 and EPGS and how to work with them in R.
R Skill Level: Beginner - you’ve got the basics of R down.
Goals / Objectives
After completing this activity, you will:
*
Things You’ll Need To Complete This Lesson
To complete this lesson you will need the most current version of R, and
preferably, RStudio loaded on your computer.
Install R Packages
- NAME:
install.packages("NAME")
More on Packages in R - Adapted from Software Carpentry.
Download Data
Set Working Directory: This lesson assumes that you have set your working
directory to the location of the downloaded and unzipped data subsets. An overview
of setting the working directory in R can be found here..
R Script & Challenge Code: NEON data lessons often contain challenges that reinforce
learned skills. If available, the code for challenge solutions is found in the
downloadable R script of the entire lesson, available in the footer of each lesson page.
Spatial-Temporal Data & Data Management Lesson Series: This lesson is part
of a lesson series introducing
spatial data and data management in R .
It is also part of a larger
spatio-temporal Data Carpentry Workshop
that includes working with
raster data in R ,
vector data in R
and
tabular time series in R .
Additional Resources
- Read more on coordinate systems in the QGIS documentation.
- NEON Data Skills Lesson The Relationship Between Raster Resolution, Spatial extent & Number of Pixels - in R
What is a Coordinate Reference System
To define the location of something we often use a coordinate system. This system consists of an X and a Y value, located within a 2 (or more) -dimensional (as shown below) space.
Coordinate Reference System Formats
There are numerous formats that are used to document a CRS. Three common
formats include:
- proj.4
- EPSG
- Well-known Text (WKT) formats.
PROJ or PROJ.4 strings
PROJ.4 strings are a compact way to identify a spatial or coordinate reference
system. PROJ.4 strings are the primary output from most of the spatial data R
packages that we will use (e.g. raster, rgdal).
Using the PROJ.4 syntax, we specify the complete set of parameters (e.g. ellipse, datum, units, etc) that define a particular CRS.
Understanding CRS in Proj4 Format
The CRS for our data are given to us by R in proj4 format. Let’s break
down the pieces of proj4 string. The string contains all of the individual
CRS elements that R or another GIS might need. Each element is specified
with a + sign, similar to how a .csv file is delimited or broken up by
a ,. After each + we see the CRS element being defined. For example
+proj= and +datum=.
UTM Proj4 String
Our project string for point_HARV specifies the UTM projection as follows:
+proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0
- +proj=utm: the projection is UTM, UTM has several zones.
- +zone=18: the zone is 18
- datum=WGS84: the datum WGS84 (the datum refers to the 0,0 reference for the coordinate system used in the projection)
- +units=m: the units for the coordinates are in METERS.
- +ellps=WGS84: the ellipsoid (how the earth’s roundness is calculated) for the data is WGS84
Note that the zone is unique to the UTM projection. Not all CRS will have a
zone.
Geographic (lat / long) Proj4 String
Our project string for State.boundary.US and Country.boundary.US specifies
the lat/long projection as follows:
+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
- +proj=longlat: the data are in a geographic (latitude and longitude) coordinate system
- datum=WGS84: the datum WGS84 (the datum refers to the 0,0 reference for the coordinate system used in the projection)
- +ellps=WGS84: the ellipsoid (how the earth’s roundness is calculated) is WGS84
Note that there are no specified units above. This is because this geographic coordinate reference system is in latitude and longitude which is most often recorded in Decimal Degrees.
Data Tip: the last portion of each proj4 string
is +towgs84=0,0,0 . This is a conversion factor that is used if a datum
conversion is required. We will not deal with datums in this particular series.
-
Read more about all three formats from the National Center for Ecological Analysis and Synthesis.
-
A handy four page overview of CRS including file formats on page 1.
EPSG codes
The EPSG codes are a structured dataset of CRSs that are used around the world. They were originally compiled by the, now defunct, European Petroleum Survey Group, hence the EPSG acronym. Each code is a four digit number.
The codes and more information can be found on these websites:
- The EPSG registry.
- Spatialreference.org
-
library(‘rgdal’) epsg = make_EPSG() # View(epsg) head(epsg)
## code note ## 1 3819 # HD1909 ## 2 3821 # TWD67 ## 3 3824 # TWD97 ## 4 3889 # IGRS ## 5 3906 # MGI 1901 ## 6 4001 # Unknown datum based upon the Airy 1830 ellipsoid ## prj4 ## 1 +proj=longlat +ellps=bessel +towgs84=595.48,121.69,515.35,4.115,-2.9383,0.853,-3.408 +no_defs ## 2 +proj=longlat +ellps=aust_SA +no_defs ## 3 +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs ## 4 +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs ## 5 +proj=longlat +ellps=bessel +towgs84=682,-203,480,0,0,0,0 +no_defs ## 6 +proj=longlat +ellps=airy +no_defs
WKT or Well-known Text
Well-known Text (WKT) allows for compact machine- and human-readable representation of geometric objects as well as to consisely describing the critical elements of coordinate reference system (CRS) definitions.
The codes and more information can be found on these websites:
##Additional Resources ESRI help on CRS QGIS help on CRS NCEAS cheatsheets