Hi,

Can someone explain to me the meaning of the following terms in detail in a layman's language w.r.t Maps !

a. Datums
b. Projections

EDIT:

Kindly do not point to Wikipedia and other similar websites, I've read that stuff but I found it to be too technical for me to understand !

What did you read up in wikipedia and other similar web sites?

A datum is something got to do with storage and projection is a term for a particular type of SQL statement.

End

They are concepts not easy to explain in few words, so inevitably I have to suggest some reading. Regarding datum you should read a geodesy manual, regarding Map Projections you have to refer to books more centered on cartography. I can suggest two publications available on-line:

1) Geodesy for the Layman, 1983 - Technical Report 80-003, Defense Mapping Agency, Washington D.C.
http://oai.dtic.mil/oai/oai?verb=get...fier=ADA142764
Chapter 4 gives a definition and an exhaustive explanation of datum. The title of this book matches exactly your requirement, doesn't it?

2) Snyder, J.P., 1987. Map Projections: A Working Manual. Professional Paper, 1395, USGS, Washington D.C.
http://pubs.er.usgs.gov/usgspubs/pp/pp1395
The first 36 pages give a good and simple introduction of the general concepts about map projections (datum included).

Cheers!

In simple terms, the datum is where you consider zero altitude to be. In the oceans you would use average sea level, but in the middle of a landmass you don't have that, so you have to define where (in the 3-dimensional Earth) you consider zero altitude.


Earth's surface is curved (approximately a sphere). Maps are flat. Projections translate between the curved surface of the Earth and the flat surface of a map. It is impossible to make a projection without introducing some distortion.

There is an extensive discussion of projections here: http://www.progonos.com/furuti/MapPr...C/cartTOC.html

Thanks for bothering to reply!

Today I downloaded both the PDFs mentioned by you.
I had a brief look at both the PDFs,
the first one looks promising
but the second one is full of math equations. I am not good at maths !

I shall read them one by one,
I hope you'll be there if I don't understand something out of them !

@cantab

Thanks for the explanation as well as the link!

I'll read it (the link) in due time and hope it gets into my head

Actually I am currently trying to render maps through Mapnik library.

Have any one of you heard about it or used it ?

Hi anishakaul, you're welcome!
Yep. Indeed it's full of maths, but I suggested this one just for the introductory part and the general concepts you were interested in. If you really need to deepen into the math formulation, it depends on what you're trying to achieve.
Unfortunately I've never heard about Mapnik before, but looking at their website it looks promising and there is some step-by-step documentation. If your only scope is to render maps, the main task is to train your mind to see the geometries and doing a lot of experiments with a well suited software is a good starting point.

You can also give a try to an open source GIS software like Grass or - maybe even better - Quantum GIS. I've still not tried them, since I'm sticked with commercial software for now, but they should not be so difficult to manage, at least for basic tasks like producing a good map using different projections and settings.

Feel free to ask. In the meanwhile... good luck!