Some years ago I wrote a paper on Quarter Degree Grid Cells. Quarter Degree Grid Cells (QDGC or QDS – Quarter degree Squares) is a way of dividing the longitude latitude degree square cells into smaller squares, forming in effect a system of geocodes. Historically QDGC has been used in a many African atlases. Several African biodiversity projects uses QDGC, among which The atlas of Southern African Birds is the most prominent one.
In this posting I will present the paper and explain the general principles of the QDGC standard. Parts of the posting is from the wikipedia article describing the standard. I wrote that article so if there is any resemblance with Wikipedia there is an explanation to it.
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. Around the equator we have 360 longitudinal lines lines, and from the north to the south pole we have 180 latitudinal lines. Together this gives us 64800 segments or tiles covering earth. The form of the squares becomes more rectangular the longer north we come. 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. 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 longitudinal and latitudinal 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.
To get smaller squares the squares are again divided in four – giving us a total of 16 squares within a degree square. The names for the new level of squares are named using a recursively reference system.
The number of squares for each QDGC level can be calculated with this formula:
number of squares = (2d)2
(where d is QDGC level)
The paper goes into detail on the standard and also places the standard in a context:
Information on the distribution of animal populations is essential for conservation planning and management. Unfortunately, shared coordinate-level data may have the potential to compromise sensitive species and generalized data are often shared instead to facilitate knowledge discovery and communication regarding species distributions. Sharing of generalized data is, unfortunately, often ad hoc and lacks scalable conventions that permit consistent sharing at larger scales and varying resolutions. One common convention in African applications is the Quarter Degree Grid Cells (QDGC) system. However, the current standard does not support unique references across the Equator and Prime Meridian. We present a method for extending QDGC nomenclature to support unique references at a continental scale for Africa. The extended QDGC provides an instrument for sharing generalized biodiversity data where laws, regulations or other formal considerations prevent or prohibit distribution of coordinate-level information. We recommend how the extended QDGC may be used as a standard, scalable solution for exchange of biodiversity information through development of tools for the conversion and presentation of multi-scale data at a variety of resolutions. In doing so, the extended QDGC represents an important alternative to existing approaches for generalized mapping and can help planners and researchers address conservation issues more efficiently.
The paper was published the 10th of March 2009 in the African Journal of Ecology and is available for download on their website. It was written together with Tomas Holmern, Steven Prager, Honori Maliti and Eivin Røskaft.
QDGC files are available for download.