This announcement has been sent to the mailing lists of "SDI-Africa", "Society of Conservation and GIS" and the "Tanzania GIS User Group".
QDGC (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 geocodes. 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 the Earth.
QDGC is currently used as a geocoding system for atlases. It has been used quite a lot on the African continent as the suitability is best between +40/-40 latitude degrees of the Equator. Examples are “The atlas of southern African birds†by Harrison et al (1997), Tanzania Bird Atlas (www.tanzaniabirdatlas.com) and Tanzania Mammal Atlas (www.tanzaniamammals.org) to have mentioned some.
The initial QDGC standard as used by Harrison et al (1997) had some shortcomings, and was subsequently updated. For more information on this work see the references listed in the end of this email.
Together with T.Holmern, H. Maliti and E. Røskaft, I am working on a paper for a more detailed presentation of this work. Should anyone have a suggestion for an appropriate journal to publish such a paper we are open for suggestions.
The presented files are a work in progress. I would particularly welcome suggestions to adjusted attributes and meta-data associated with the files. For references use the QDGC website until further notice. The full set will be associated with the afore mentioned paper.
Last update of the QDGC files were made in January earlier this year (2007). Improvements were made based on these pending issues:
- slightly inaccurate center points
- no corner coordinates
- separate files for separate countries
- inadequate file nomenclature
This has now been fixed for QDGC level 1 and 2.
The users should note that the only squares exported for each country file are those in part or completely overlapping with the country shape file (the technical term is "intersect"). Some of the squares intersecting the border would thus not be completely within the related country's border and thus shared with the neighboring country.
A list of the new files are available on these pages at www.qdgc.org:
Browsing in the file repository is available here:
To facilitate the identification of the squares and ease the use of them in different contexts several attributes are provided with the files:
|
Atribute |
Description |
|
ARAEA |
Calculated with XTools Km2, projected Coordinate System type, Albers Equal-Area? Conic Projection (ref: ArcGIS and Wikipedia http://en.wikipedia.org/wiki/Albers_projection) |
|
QDGC |
QDGCc string eg
S01E003A? |
|
LON_C |
Square centerpoint longitude
calculated With XTools |
|
LAT_C |
Square centerpoint latitude
calculated With XTools |
|
LON_NW |
Square north-western longitude |
|
LAT_NW |
Square north-western latitude |
|
LON_NE |
Square north-eastern longitude |
|
LAT_NE |
Square north-eastern latitude |
|
LON_SE |
Square south-eastern longitude |
|
LAT_SE |
Square south-eastern latitude |
|
LON_SW |
Square south-western longitude |
|
LAT_SW |
Square south-western latitude |
Figure: QDGC shapefiles level 2 reference E030S10CB?.
More information and links to the different national files are available from the QDGC website:
Further work based on the QDGC is being planned:
- Steven D. Prager with the University of Wyoming has helped me with a script to speed up the process of generating later QDGC updates. Minor edits to the scripts will be made.
- Prepare files for QDGC levels 3-5 for each country.
- Update the QDGC website adding the lonlat2qdgc tool (python script).
- Remove QDGC related files from the TZGISUG website
- Make a .kml/kmz file based on the shape files.
- Do the last 3-5 days of work on the paper and submit it to a journal to document what has been done.
- I am looking into the possibilities of integrating the QDGC files in an African continent stand-alone atlas system using HostGIS, MySQL and PHP. This could prove to be a useful tool for baseline biodiversity studies.
- I have set up a CVS (version repository) for my own use on a server. There is a possibility I might open it for anonymous read access.
- Check if other countries around the Equator could find this geocoding standard useful.
The work done on the QDGC files so far has not been explicitly funded. As such it has relied on me being able to do this on my spare time. Should anyone feel that this work deserves some funding then just send me an email for more details.
Money is not everything, and the work so far would not have been possible without the skillful help from officers at the Tanzania Wildlife Research Institute, Dept of Biology at Norwegian University of Technology and Science, Tanzania Mammal Atlas Project, Tanzania Bird Atlas, Julian Blanc, Howard Frederick, Grant Hopcraft and a lot more. The Avian Demography Unit at University of Cape Town deserves a special mention for being the first ones to actually document the use of QDGC/QDS.
Again - should you have any comments, questions or advice related to my work on QDGC please send me an email.
Regards,
Ragnvald Larsen
Harrison JA, Allan DG, Underhill LG, Herremans, M, Tree AJ, Parker V, Brown CJ (eds) 1997. The atlas of southern African birds. Vol. 1: Non-passerines. Vol. 2: Passerines. Johannesburg: BirdLife? South Africa.
Larsen, R. 2006. Web mapping biodiversity. Website article at Tanzania GIS User Group website (http://tzgisug.or.tz).
Larsen, R. 2006. Bridging data repositories - introducing lonlat2qdgc.exe. Website article at Tanzania GIS User Group website (http://tzgisug.or.tz).
Larsen, R. 2006. Generating QDGC files (Updating QDGC revisited). Website article at Tanzania GIS User Group website (http://tzgisug.or.tz).
Larsen, R., Mduma, S., Maliti, H., Kaaya, J., Machoke, M. and Frederick, H. 2005. Generalisation of spatial wildlife data in Tanzania. Arusha. TAWIRI conference.
Wikipedia. http://en.wikipedia.org/wiki/QDGC