Question 4: Road Extraction

[Part 1] Getting a raster road map

Method

1. Go to “Tools” to open “Algorithm Librarian”
2. Click the “Find” button and type “THR” (Thresholding Image to Bitmap) and open the algorithm;
3. Enter the threshold minimum (58) and maximum (160) pixel value
4. “Run” the algorithm.

 Result

Road Extraction image with Threshold Min: 58, Threshold Max: 160

Analysis

- Thresholding is the easiest or simplest method of creating a binary layer to extract the roads from the rest of the image. Different threshold minimum and maximum number changes clearness of roads and the amount of built-up area. Although we are able to extract the roads, there are needs to clean up the classes that has pixel value falling into threshold 58 to 160. (Such as built-up area and other land cover types will be included).  A more elaborate and concise tool may be used.

 [Part 2] A map after eliminating spurious noise

 Methods: 
Within the raster format, we will be able to edit delete spurious noise and add more road if needed. In order to digitize the map by hands, we might need to have true colour map for comparison.
 Analysis:
This method is time consuming; however, by doing this work, we can get more clear road feature in the image.





 [Part 3] How to Make the Road-line Clear

 
Methods:

From the Algorithm Librarian, Find BIT2Line This module converts a bitmap layer to a line layer.

Analysis:

This Algorithm make road line more clear. It converted image type from raster to vector, and this module converts a bitmap layer to a line layer. By operating this algorithm, little noises that are not caught as a line are eliminated, so this module clears small noises that human made during digitizing.

[Part 4] Overlaying with True Colour Image

Analysis:

The true colour map, which is already filtered, can be overlayed with the extracted road map. This tells us road feature of the area.

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