Raster Data Model

A simple, storage intensive format best suited for continuous fields.

Raster Data Model

Represents space continuously:

  • Rectangular grid of equally sized cells
  • Each cell only has one value

Raster Data Model

Multiple attributes, require multiple bands

  • Standard color photos have three bands:
    • Red, green, & blue
  • LANDSAT8 has 11 bands:
    • "False Color Images"
    • Spectral Indices (NDVI)

Resolution vs. Extent

Resolution: (aka cell size) is the smallest resolvable feature.

  • 1 m x 1 m = 1 m2

Extent: (aka domain) depends on number of rows and columns along with the cell size.

  • 5 rows x 5 columns = 25 cells
  • 1 m cell size
  • Covers 5 m x 5 m = 25 m2

Resolution vs. Extent

Resolution: 10 m cell size = 100 m2 area per cell.
Extent: varies with number of cells and cell size.

Rows Columns Extent
5 5 50 m x 50 m = 2,500 m2
5 10 50 m x 100 m = 5,000 m2
100 100 1,000 m x 1,000 m = 1,000,000 m2

TopHat Question 1

Raster resolution is dictated by the number of rows and columns, raster extent is dictated by the cell size.

  • True
  • False
  • Sometimes True, Sometimes False

Mixed Pixel Problem

One cell: one value

  • What if it covers multiple values?
    • A: Winner take all
    • B: Cell center
  • Other options?
    • Containment - if a value of interest is anywhere within a cell
      • e.g. invasive species presence

Implications

Loss of information during rasterization.

  • A "bigger" issue for larger cells.
  • At a certain point features become unrecognizable.
  • Higher resolution = larger file for equivalent areas.

TopHat Question 2

This term describes how information is lost when creating a raster image.

  • Resolution
  • Domain
  • Mixed pixel problem
  • Rasterization

File Size

The number of cells per image dictates file size.

  • Cells = rows x columns x bands
  • Increases exponentially with resolution
    • For a given extent
  • Increases linearly with number of bands
Extent Bands Cell Size Number of Cells
1x1km 1 100m 100
1x1km 1 1m 1,000,000
1x1km 3 100m 300
1x1km 3 1m 3,000,000

Why does this matter?

Downloading, uploading, and processing times will all increase as the file size increases.

  • Large raster datasets are computationally "expensive"
    • More cells = more "overhead"
  • Practically speaking, how will this impact you?

TopHat Question 3

You downloaded a 3 band LANDSAT8 image covering the UBC campus. The image's dimensions are 6000m (rows) by 4500m (columns) and the resolution (cell size) is 30m. How many cells does the image contain?

  • 30,000
  • 90,000
  • 900,000
  • 2,700,000
  • 27,000,000
  • 81,000,000

Metadata in ArcPro

Raster Overlay

Raster data can be used to evaluate algebraic expressions easily and efficiently.

  • A: Winter Temperature
  • B: Summer Temperature
  • Range: B-A
  • Average: (A+B)/2

Raster Calculator

The tool for this in Arc is called the "Raster Calculator"

  • Raster layers can be "variables" in linear equations
  • Perform simple calculations incorporating one or multiple layers
  • Ideally, layers should have the same resolution and extent
    • Resampling is required if not
    • Output will be restricted to "smallest" extent

Key Advantages

The raster data model is well suited for continuous phenomena:

  • Continuous in space
    • Complete coverage within the domain (extent), everywhere has a value
  • Continuous in space and time
    • Bands can be used to represent time steps
    • Common tool approach for climate data
      • i.e., bands 1, 2, 3 ... could represent mean Jan, Feb, Mar temperatures
  • Simple data structure makes raster overlay easy and efficient
    • Multiple raster layers can be combined and evaluated using expressions
    • Easy to "show" others what has been done

GeoTIFF

One of the most common/functional raster formats, based of the Tag Image File Format (TIFF). A TIFF file stores metadata (data about the data) as tags. A GeoTIFF is a standard .tif image format plus additional tags spatial tags denoting spatial information including:

  • Extent (minimum x,y and maximum x,y)
  • Resolution (cell size)
  • Projection, Coordinate system, and datum

Other file types

Raster data can come in many different formats. You will likely encounter when working with raster data include:

  • IMG - A proprietary image format commonly used by ESRI products
  • JPEG2000 - A geospatial version of the common .jpg image type
  • ASCII - An older human readable format (simple text file) with slower performance than the types listed above

TopHat Question 4

The Raster Calculator tool lets you:

  • Evaluate algebraic expressions using raster layers as inputs
  • Create raster layers
  • Calculate the area of raster layers
  • Overlay raster and vector layers