Raster data

Question 1

What are the 5 conceptual steps to creating a raster file?

Answer 1

• Choose grid resolution (1/2 the length and 1/4 the area of the smallest feature to map)
• set data type (integer/real)
• overlay grid over study area
• assign attribute code to each grid cell
• repeat process for each map layer

Question 2

What are the 5 ways we can input raster data into a GIS?

Answer 2

• Manual raster coding
• Raster scanning (aerial photos/ hard copy maps)
• Existing digital raster data
• Remote sensing imagery
• Vector to raster conversion

Question 3

What are the steps of vector to raster conversion?

Answer 3

• Code polygons
• Overlay grid cells on top of polygons
• assign attribute code to each polygon

Question 4

How do we convert raster to vector?

Answer 4

• Boundaries are set up between different attribute classes

- A polygon is created by storing x and y coordinates for the points adjacent to the boundaries

Question 5

In terms of cell values, grid cell representation is often used for _ data and _ data

Answer 5

• Categorical (nominal or ordinal)

- quantitative (interval or ratio)

Question 6

You can code data as

Answer 6

• Integers
• Real values (decimals)
• Alphabetic values

Question 7

What are the 4 methods of encoding?

Answer 7

• Presence/absence (if a single feature even touches the cell, the cell is that attribute); for polygons, the polygon which covers the highest # of cells is recorded
• Centroid of cell: something’s only recorded when it touches the centre of the cell (only good for areal data/ continuously variable quantities like elevation)
• Dominant type: encodes the presence of something if it takes up more than 50% of cell (most common method)
• percent occurrence

Question 8

what can raster data represent?

Answer 8

• Visual images
• discrete value
• continuous value
• null data

Question 9

What are methods for organizing multiple raster layers?

Answer 9

• Band interleaved by line (BIL)
• Band interleaved by pixel (BIP)
• Band sequential (BSQ)

Question 10

Explain BSQ

Answer 10

• stores each characteristic in separate file
• good for compression
• bad if focus on 1 area

Question 11

Explain BIP

Answer 11

• all values for a pixel grouped together
• good for focusing on multi-area characteristics
• bad if want to remove or add a layer

Question 12

T or F, it’s impossible to convert between BIP, BIL, and BSQ.

Answer 12

F

Question 13

What are the 4 data compression methods?

Answer 13

• Run length encoding
• Chain encoding
• Block encoding
• Quadtrees

Question 14

What is the limitation that run length encoding has?

Answer 14

-File is read left to right, one row at a time

Question 15

How does chain encoding work?

Answer 15

• Scans rows and columns to define 2D regions with same cell values
• starting point (origin) and direction to follow around the boundary

Question 16

How does block encoding work?

Answer 16

• Areas of common cell values are represented with single value
• square blocks used to tile the area to be represented (blocks of the largest size possible)
• store top left corner

Question 17

What are the 4 disadvantages of raster data structure?

Answer 17

• Reduced spatial accuracy of discrete objects
• need for large storage capacity
• some spatial relationships (contiguity and connectivity) may be altered or lost

Question 18

Explain quadtrees

Answer 18

• Keep dividing the grid into quarters until you have the same attribute for the square.
• good for homogeneous areas

Question 19

what are some advantages of raster data structure?

Answer 19

• abundant data sources
• raster algorithms are often computationally simpler and faster than vector
• better for modelling continuous features than vector and analyses that involve spread, flow, or diffusion processes (i.e. surface modelling, overlay)