GIS Methodology

Making the movie Automatic vectorization Go to Sundaland menu

Making the movie The first step in making a movie of any kind is to assemble a series of frames. The frame on the right is the last of a series of 43 bitmap images (BMP) produced using IDRISI. The Digital Elevation Model DEM provides a flexible tool for analysis. The data used was the ETOPO 5-minute DEM from the National Geophysical Data Center (NGDC). Frame zero represents present sea level and each subsequent frame represents a 5-meter fall in sea level to 110 meters below the present level (frame 43 at the right). How did I arrange to show movement of sea level? IDRISI uses palette files to assign colors to the numerical value stored for each picture element (pixel), here elevation.

I reclassified all elevations in steps of 5 meters and then assigned colors to the steps. The first frame shows blue for the surface of the sea at its present level. For each subsequent frame I made a new palette file showing blue 5 meters lower. Colors for land above the present sea level do not change since this might give the visual signal that the land was moving vertically. There are many programs that convert a series of frames into a video, a subject that I do not discuss here. The thin black line outlines the modern coastline. It is a vector overlay and is a critical part of the movie but was not included with the original data. I did not have to trace out thousands of kilmeters of the coastline by hand because IDRISI did it automatically as explained in the next section. Go to top of page Making vector overlays automatically with IDRISI I needed to make a vector file to show the modern coastline for a visual reference in the movie. Built-in Facility for Digitizing IDRISI has limited facilities for making vector files. There is a built-in rudimentary routine for manually digitizing points, lines, and polygons. However, to digitize regularly one should buy Clark Labs utility, CARTALINX. Third-Party Software Alternatively, there is R2V (from www.ablesw.com), a fully functional free demo that is capable of dealing with raster images up to 512X512 pixels. (One can trim down to 512 if the area to be vectorized is only a small part of the study area and or vectorize a map in segments if none of the polygons have to be split.) Bigger than that one needs to buy the R2V product. Automatic Vectorization IDRISI does have the capability to vectorize automatically and I decided to use this capability instead of digitizing the coastline manually. To vectorize a coastline we need to produce a boolean raster file showing the elevation of the water plane as 0 and the rest of the image as 1. (Or vice-versa). Then we use REFORMAT / Raster to Vector conversion. Either a line vector or a polygon vector file can be used. To get a line or polygon vector we need to define a feature so that there is a line for IDRISI to follow. Using RECLASS, make a Boolean image (0,1) such that the boundary between 0 and 1 defines a line. Conceptually this is a black and white image with the boundary between white and black defining the line to be expressed as a vector. Possible glitch: A very complex coastline and/or a water body containing a lot of tiny islands may need smoothing using FILTER. When I applied the method to the area of Red Sea-Horn of Africa no smoothing was needed, but when I applied it to the whole of SE Asia I had to use a lot of smoothing to eliminate some of the islands that appeared as not much more than specks. I suggest trying out the method using a small area of a DEM to gain confidence in its utility. Details for making a vector polygon file from a DEM (raster file). Vector line files can be produced using a similar method. Run: REFORMAT / Raster/Vector Conversion / POLYVEC Selecting: IDRISI polygon file; input image; output image; Ticking: Exclude a background polygon = 0 Display: Open DEM image; add as a layer Ir new polygon vector file; In Composer select Ir vector file and Click Layer Properties Click Symbol file Select idrpoly otherwise there will be just a black rectangle to look at. Go to top of page