Raster Data

• The spatial raster data model represents the world with the continuous grid of cells (a.k.a. pixels)
• This data model often refers to so-called regular grids, in which each cell has the same, constant size
• We will focus on the regular grids only. However, several other types of grids exist, including rotated, sheared, rectilinear, and curvilinear grids (see Chapter 1 of Pebesma and Bivand (2023)).

Figure 4.1: Cell offset from origin (lower left)

Figure 4.2: Cell values (for example elevation)

Figure 4.3: Color representation of cell value

Types of raster data

• Raster datasets usually represent continuous phenomena such as elevation, temperature, population density or spectral data.
• Discrete features such as soil or land-cover classes can also be represented in the raster data model

A simple example: Elevation

      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11]
 [1,]  275  282  373  342  357  326  372  318  400   243   303
 [2,]  230  318  316  351  345  346  359  331  395   225   288
 [3,]  164  337  258  342  363  350  349  320  395   280   321
 [4,]  168  337  261  354  358  364  339  377  368   309   284
 [5,]  202  322  250  380  362  373  327  393  360   379   326
 [6,]   NA  310  270  361  370  363  368  368  385   383   297
 [7,]   NA  277  310  291  375  365  375  355  343   407   220
 [8,]   NA  181  325  264  381  373  389  341  305   395   252
 [9,]   NA   NA  313  264  370  384  392  328  357   376   289
[10,]   NA   NA  298  285  370  380  386  354  349   385   311
[11,]  402   NA  333  293  356  382  376  391  329   352   361

A more complex example: RBG Image

• A lot of raster data is obtained using remote sensing (RS)
• Typically, RS imagery consists of more than 1 band
• In this case, the data is stored in a 3 dimensional array (where band is the 3rd-dimension)
• A RS image can contain any number of bands.
• The most well known type of RS imagery consists of 3 Bands in the red, blue and green spectrum

Figure 4.4: The bands stored internally

Figure 4.5: The RGB Composite

Each band is a 2D matrix

Figure 4.6: Band values

Figure 4.7: RGB True Color Image

Multispectral Datasets

• Multiband datasets usually capture different parts of the EM spectrum
• E.g. the Landsat image from the previous example has 6 bands capturing the following wavelengths:
  • Band 1: Blue (0.45 - 0.52 µm)
  • Band 2: Green (0.52 - 0.60 µm)
  • Band 3: Red (0.63 - 0.69 µm)
  • Band 4: Near-Infrared (0.77 - 0.90 µm)
  • Band 5: Short-wave Infrared (1.55 - 1.75 µm)
  • Band 7: Mid-Infrared (2.08 - 2.35 µm)

NirGB Image

Figure 4.8: Band values

Figure 4.9: NIR-GB False Color Image

Representations of multispectral data

• A true color image is created by using the Red (3), Green (2) and Blue (1) Band and mapping these to RGB
• A false color image is created by mapping other bands to RGB

Figure 4.10: The R, G and B bands mapped to RGB

Figure 4.11: NIR, G and B bands mapped to RGB

Pebesma, Edzer, and Roger Bivand. 2023. Spatial Data Science: With Applications in R. Boca Raton: Chapman; Hall/CRC. https://doi.org/10.1201/9780429459016.