Thursday, May 15, 2008

Why do we use a Geographic Information System?

In a recent post we described how we manually converted a high-resolution aerial image mosaic of the Bermuda reef platform into a set of georeferenced polygons - creating a digital map of all the reefs around the island.

There are many benefits to having an electronic map instead of a paper map.

Identity
When each patch reef was added to the database, they were each assigned a number. This means every single patch reef in Bermuda now has a unique ID tag that can be used to name them.

In this section of the aerial image, abouto 1km wide,
you can see the ID number of each patch reef.

Having an ID number for every reef helps scientists and resource managers, as it gives another way to distinguish each reef.

Proximity Mapping
We can ask the GIS programme to select reefs within a certain distance of another mapped object. For instance, if we have a layer representing the Island, we can ask the programme to select and highlight all of the reefs that are within 1 nautical mile from shore. This then gives us a map of all of the reefs where spear fishing is not allowed. We can also then ask the computer to give us a data table of the ID numbers of each reef within this protected zone.

A map of all the patch reef in the No Spearing Zone.

Size, Shape, Isolation, etc etc...
We can also get the programme to highlight patch reefs based on a wide range of other parameters, many of which it calculates from the polygons around each reef. This means we can get data from the map that we did not have before we made it.

For instance, we can get the programme to select reefs of a specific size, or only reefs that are round, or reefs that are near other reefs. We can then use this information to guide research: Reefs of different sizes may contain different numbers of fish species. Round reefs have a smaller edge exposed to the surrounding water compared to an elongated reef, and therefore might have alternate rates of larval fish and coral recruitment. Reefs near other reefs also may differ in fish and coral assemblages relative to reefs that are isolated.

A map of patch reefs in White Flats colour-coded by size (area). Click image to enlarge

So - why do we use the GIS maps instead of paper?

Because paper maps are static, but GIS maps are DYNAMIC!!!