Week4: 10 Data conversion

Question 1

Vector data vs Raster data

Answer 1

Vector data: focuses on spatial objects which are explicitly stored
Raster data: focuses on the underlying space. Objects must be “extracted”

Question 2

Vector data structures vs Raster data structures

Answer 2

Vector: topology is explicit - a subset of the topological relations is explicitly stored
Raster: topology is implicit - topological relations must be calculated (generally by considering minimum bounding rectangles (MBR) of objects)

Question 3

Raster-to-Raster

Answer 3

1) Raster formats differ in the way they are stored
2) Conversion between raster formats requires reorganisation of the raster cells & their corresponding data values (relatively simple operation)

Question 4

Vector-to-Vector

Answer 4

1) Different vector formats are based on different data structures
2) Converting from one format to another requires converting from one data structure to another (a bit more tricky but straightforward)

Question 5

Raster-to-Vector

Answer 5

To extract a non-orthogonal vector with unit width from the corresponding raster structure, the 4-connected approach results in distortion because such a vector can only be represented by horizontal and vertical line segments, which lie in the general direction of the vector as fragmented segments

Question 6

Vector-to-Raster

Answer 6

involves overlaying the vector to a raster array and identifying the pixels that the vector intersects.

• this approach often produces a stair-step distortion (aliasing)
• Antialiasing: gray scale pixels according to coverage measures of the pixel by the vector (e.g., length of the intersection)