GIS Lecture 13: Data Modelling

GEOG 201 with Professor Tim Elrick. Prep for Midterm II. Lecture 13: Data Modelling

Order of data models

From Reality →
Geospatial (how to conceptualize, into geospatial data)
Conceptual (how to model - what attributes)
Logical (how to use in software - table structure, field-based or object, etc.)
Physical (how to implement/store - which software and file type)

Increasing abstraction
From human- to computer-oriented

Single-band and multi-band types for Raster data in field-based modelling for logical/physical model stage

Single band

• Binary

  • 1s and 0s

• Grayscale

  • Shades of gray 0-255

• Display colourmap image (with accompanying colourmap)

  • RGB 0-255

Multi-band

• One band each for R, G, and B

• Pixels with 0-255 for the colour

• Bands overlay to produce final product

Storage methods for Multiband Raster Data

• BIL

  • Bands Interleaved by Line

  • One row at a time, all 3 bands at once

• BIP

  • Bands Interleaved by Pixel

  • All 3 bands at once, this time by column (each pixel)

• BSQ

  • Band at a time, all pixels for each band

ASCII Header

7-bit
128 states - numbers, letters, special characters
used for raster storage
describes data (# of rows and columns, value range, etc.)

Bit

1 bit is one data unit
Binary: Can take form 0 or 1
Data with n bits can take 2^n states

8 bits is 1 byte

Georeferencing

Attaching location to geographic information
- For vector data: convert coordinates to new CRS

- For raster data: CRS refers to specific corner of grid, with specified resolution

Lossless compression

• Recovers data fully

• Up to 5x smaller

• Run-length compression

• Quad-tree method

Lossy compression

• Cannot recover 

• Up to 40x smaller

• wavelet method

Elementary Data Types

• Character

• Integer (including binary)

• Single

  • Floating point, 32-bit

  • 7-8 sig figs

• Double

  • Floating point 64-bit

  • 15-16 sig figs

Extended Data Types

• Array

  • set of elementary data

  • 2D array = matrix

• String

  • set of characters

  • letters to sentences

• BLOB

  • binary set of length n

  • Used to save whole files/images

Physical data storage - Differentiation