Thursday, November 1, 2007

Quarter Degree Grid Cells: Another way of Mapping Africa

Recently, Ragnvald Larsen of the Norwegian University of Science and Technology (NTNU) in Trondheim released some news to the Spatial Data Infrastructure for Africa (SDI-Africa) mailing list.

He has been working on project about creating Quarter Degree Grid Cells for mapping purposes for the African countries on a national level.

But what are Quarter Degree Grid Cells?

Quarter Degree Grid Cells (QDGC) are a way of dividing longitude and latitude degree square cells into smaller squares, forming in effect a system of geocodes. This is similar to the NTS system in Canada (for mapping the Northern areas of Canada) and to the way the North Sea is mapped when determining leases by the various countries involved (Norway, Denmark, UK, Germany, the Netherlands). An example of how the North Sea is subdivided by country follows:

The respective sectors are divided by median lines agreed in the late 1960s.

In the United Kingdom, the UKCS (United Kingdom Continental Shelf) is divided into quadrants of 1 degree latitude and one degree longitude. Each quadrant is divided into 30 blocks measuring 10 minutes of latitude and 12 minutes of longitude.

Norway has a similar model and is divided into quadrants of 1 degree by 1 degree. Norwegian licence blocks are larger than British blocks, being 15 minutes of latitude by 20 minutes of longitude (12 blocks in a quad).

In Denmark, the Danish sector of the North Sea is divided into 1 degree by 1 degree quadrants, and their blocks are 10 minutes latitude by 15 minutes longitude.

Germany and the Netherlands share a quadrant and block grid - quadrants are given letters rather than numbers. The blocks are 10 minutes latitude by 20 minutes longitude. The Dutch sector is located in the Southern Gas Basin and shares a grid pattern with Germany

So the theory of using grid squares has been around for quite some time.

Almost Equal Areas

QDGC represents a way of making (almost) equal area squares covering a specific area to represent specific qualities of the area covered. The squares themselves are based on the degree squares covering earth.

We know that around the equator there are 360 longitudal lines lines. For latitude, i.e. from the north to the south pole we have 180 latitudal lines. Multiplying we determine that this gives 64800 segments or tiles that can cover earth. The form of the squares becomes more rectangular the further north or south we move. At the poles they are not square or even rectangular at all, but end up in elongated triangles.

Each degree square is designated by a full reference to the main degree square.

An Example using Tanzania

Taking from the project web-site, I'll use their example, with regards to Tanzania.

S01E010 is a reference to a square in Tanzania. S means the square is south of equator, and E means it is East of the zero meridian.

The numbers refer to longitudal and latitudal degree.

A square with no sublevel reference is also called QDGC level 0.

This is square based on a full degree longitude by a full degree latitude. The QDGC level 0 squares are themselves divided into four.

A grid at this level is shown as:


Smaller squares are determined by dividing the above squares into 4 again.

So if we divide S01E010 by four again, the new grid would be S01E010AD.

The number of squares for each QDGC level can be calculated using the following formula:

number of squares = (2^d)^2 where d is QDGC level

Putting all the above theory into place, there is code out there that will allow you to compute a Quarter Degree Grid Cell, please follow the link here.

Project Information

For more information on this project and the work done by Ragnvald Larsen, please take a look at QDGC.

The attached image shows QDGC being used for mappings of the fires in Africa in the year 2000.