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Spatial Resolution & Spatial Extent

A raster consists of a series of pixels, each with the same dimensions and shape. In the case of rasters derived from airborne sensors, each pixel represents an area of space on the Earth’s surface. The size of the area on the surface that each pixel covers is known as the spatial resolution of the image. For instance, an image that has a 1 m spatial resolution means that each pixel in the image represents a 1 m x 1 m area.

The spatial resolution of a raster refers the size of each cell (in meters in this example). This size in turn relates to the area on the ground that the pixel represents.
A raster at the same extent with more pixels will have a higher resolution (it looks more "crisp"). A raster that is stretched over the same extent with fewer pixels will look more blury and will be of lower resolution.

Raster Extent

To be located geographically, the image's location needs to be defined in geographic space (on a spatial grid). The spatial extent defines the 4 corners of a raster within a given coordinate reference system.

Coordinate Reference Systems

R Functions

# reproject a vector object:
spTransform(vectorObject, crs)

# reproject a raster objects
projectRaster(raster, crs)

# set crs using EPSG code
CRS("+init=epsg: 32611")

Getting Started with CRS

Check out this short video highlighting how map projections can make continents look proportionally larger or smaller than they actually are!

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 space.

We use coordinate systems with X, Y (and sometimes Z axes) to define the location of objects in space. Source: http://open.senecac.on.ca

While the above coordinate system is 2-dimensional, we live on a 3-dimensional earth that happens to be “round”. To define the location of objects on the earth, which is round, we need a coordinate system that adapts to the Earth’s shape. When we make maps on paper or on a flat computer screen, we move from a 3-Dimensional space (the globe) to a 2-Dimensional space (our computer screens or a piece of paper). The components of the CRS define how the “flattening” of data that exists in a 3-D globe space. The CRS also defines the the coordinate system itself.

A CRS defines the translation between a location on the round earth and that same location, on a flattened, 2 dimensional coordinate system. Source: http://ayresriverblog.com

A coordinate reference system (CRS) is a coordinate-based local, regional or global system used to locate geographical entities. – Wikipedia

The Components of a CRS

The coordinate reference system is made up of several key components:

  • Coordinate System: the X, Y grid upon which our data is overlayed and how we define where a point is located in space.
  • Horizontal and vertical units: The units used to define the grid along the x, y (and z) axis.
  • Datum: A modeled version of the shape of the earth which defines the origin used to place the coordinate system in space. We will explain this further, below.
  • Projection Information: the mathematical equation used to flatten objects that are on a round surface (e.g. the earth) so we can view them on a flat surface (e.g. our computer screens or a paper map).

Why CRS is Important

It is important to understand the coordinate system that your data uses - particularly if you are working with different data stored in different coordinate systems. If you have data from the same location that are stored in different coordinate reference systems, they will not line up in any GIS or other program unless you have a program like ArcGIS or QGIS that supports projection on the fly. Even if you work in a tool that supports projection on the fly, you will want to all of your data in the same projection for performing analysis and processing tasks.

Data Tip: Spatialreference.org provides an excellent online library of CRS information.

Datums

Another nice explanation of projections and datums.

UTM Zones

A CRS defines the translation between a location on the round earth and that same location, on a flattened, 2 dimensional coordinate system. Source: http://ayresriverblog.com