Spatial Thinking in Planning Practice: An Introduction to GIS

6 RASTER DISADVANTAGES: 1. Appearance . Cells “seem” to sacri!ce too much detail (Figure 1.9). "is disadvantage is largely aesthetic and can be remedied by increasing the layer’s resolution. 2. Accuracy . Sometimes accuracy is a problem due to the pixel resolution. Imagine if you had a raster layer with a 30 by 30 meter resolution, and you wanted to locate tra&c stop signs in that layer. "e entire 30 by 30 meter pixel would represent the single stop sign. If you converted this raster layer to vector, it might place the stop sign at what was the pixel’s center. Sometimes problems of accuracy (and appearance) can be resolved by selecting a smaller pixel resolution, but this has database consequences. 3. Large database . As just described, accuracy and appearance can be enhanced by reducing pixel size (the area of the Earth’s surface covered by each cell), but this increases your layer’s !le size. By making the res- olution 50 percent better (say from 30 to 15 meters), your layer grows four times. Improve the resolution again by halving the pixel size (to 7.5 meters) and your layer will again increase by four times (16 times larger than the original 30-meter layer). "e layer quadruples because the resolution increases in both the x and y direction. Figure 1.4. Visual depiction of overlay analysis. Source: ESRI. http://www.esri.com/news/arcnews/fall04articles/ arcgis-raster-data-model.html MORE ON VECTOR DATA MODELS "e real world is too complex and unmanageable for direct analysis and understanding because of its countless variability and diversity. It would be an impossible task to describe and locate each city, building, tree, blade of Chapter 1: De!ning a Geographic Information System