- Quarter Degree Grid Cells (or QDS - Quarter degree Squares) are a way of dividing the longitude latitude
degree square cells into smaller squares, forming in effect a system of
. Historically QDGC has been used in a lot of African atlases. Several African biodiversity projects uses QDGC, among which The atlas of Southern African Birds
[HARRISON, J.A., ALLAN, D.G., UNDERHILL, L.G., HERREMANS, M., TREE, A.J., PARKER, V. & BROWN, C.J. (1997) The atlas of southern African birds. Vols 1 and 2, BirdLife South Africa, Johannesburg, South Africa.]
is the most prominent one. In 2009 a paper by Larsen et al.
[R. Larsen, T. Holmern, S. D. Prager, H. Maliti and E. Røskaft (2009) Using the extended quarter degree grid cell system to unify mapping and sharing of biodiversity data, African Journal of Ecology. Volume 47, Issue 3 , Pages382 - 392]
describes the QDGC standard in detail.
The squares themselves are based on the degree squares covering earth. QDGC represents a way of making approximately equal area squares covering a specific area to represent specific qualities of the area covered. However, differences in area between 'squares' enlarge along with longitudinal distance and this can violate assumptions of many statistical analyses requiring truly equal-area grids. For instance species range modelling or estimates of ecological niche could be substantilly affected if data were not appriopriatelly transformed, e.g. projected onto a plane using a special projection.
[Elith, J., Phillips, S. J., Hastie, T., Dudík, M., Chee, Y. E., & Yates, C. J. (2011). A statistical explanation of MaxEnt for ecologists. Diversity and distributions, 17(1), 43-57.]
Around the equator we have 360 longitudinal 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 above 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 the same way. The full reference of a square could then be:
The number of squares for each QDGC level can be calculated with this formula:
number of squares = (2d)2
(where d is QDGC level)
Table showing level, number of squares and an example reference:
To decide which name a specific longitude latitude value belongs to it is possible to use the code provided on this Github project:
Download shapefiles datasets here:
Digital orthophoto quadrangle