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I am working on a project to try to identify the most threatened populations (on a global scale) of having a limited access to freshwater. I want to compare those most at risk currently and then add in climate change projections to see how it is expected to change. I am trying to justify what projection system to use. With such a global scale what is the best option, if focusing on freshwater geography as well as human demographics? I am a little overwhelmed, but looking at mostly WGS 1984 coordinate systems. How would you justify a certain projection for such a broad analysis area and concentrations.

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Viewing anything on a global scale suits geographic projection, WGS84 being one of them. It is a difficult to get scale and measure from geographic data but on this broad scale that wouldn't be the intent. The data remains the same regardless of projection and most modern spatial software will allow analysis and mapping on geographic data without problems. –  Michael Miles-Stimson Jun 2 at 3:39
    
So really it doesn't matter as long as it is a global projection? How can I be asked to justify which projection I use then? –  Dana Jun 2 at 5:28
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There are really three separate questions here: (1) What coordinate system to use for storing data? (2) What coordinate system(s) to use for the analysis? (3) What map projections to use for display of geographic features? All three can differ. Which of these questions are you trying to ask? If it is (2) or (3), please edit your question to explain the methods or purposes of your analysis. If it is (3), please tell us what information the map should convey and help us understand the intended audience. –  whuber Jun 2 at 18:20

2 Answers 2

If you are looking at demographic, I suggest that you use some kind of equal area projection. This will give to each country a size that reflects its true size.

There are several equal area projection, including sinusoidal or cylindrical equal area. Cylindrical equal area is quite distorted, but it is convenient for WebGIS because you can seemlessly turn around the world. However, for a static map you could look at sinusoidal or Mollweide (see a list here) which are less distorted.

Note: With Plate carree (the "default" projection with lat/long coordinates) or Mercator (to give another example), you will increase the importance of countries located at high latitudes, which are mainly located in the northen hemisphere ( there is not much inhabitated land in the Southern high latitude compared with the Northern hemisphere). Because countries of the high latitude usually suffer less from freshwater issues, you will give a more positive signal.

remark : you could also look at the more fancy Waterman projection (a nice compromise between conformity and equal area, and a nice projection name for freshwater mapping)

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I take exception to the statement that there's not much inhabited land in the southern hemisphere. The Northern hemisphere is mostly 1st world and freshwater could/is piped to where it needs to be from where it's plentiful. It's Africa, South America, India.. that would suffer, lacking the infrastructure and wealth to implement such projects. All of these are (mostly) in the southern hemisphere. That aside you make a good point, there is much more habitation and infrastructure north of the equator. –  Michael Miles-Stimson Jun 2 at 22:00
    
Most of Asia (including India and China) and 2/3 of Africa (including Magreb + Sahara + Sahelian countries) are actually in the Northen hemisphere. What I meant is that it does not really matter if you distort the area in Antartica, but it does for the high latitude of the Northern hemisphere. –  radouxju Jun 3 at 6:25
    
You are correct. There is less land mass and much less population and infrastructure in the south but as a person who lives south of the equator I can assure you that there is population here and shouldn't be dismissed or diminished. I think that whuber has it correct in their comment. –  Michael Miles-Stimson Jun 3 at 21:55
    
agree. This is why I suggested to use an equal area projection. I've edited my post to (hopefully) avoid ambiguities. –  radouxju Jun 4 at 10:57

I would portray the output data on Mollewide or Robinson.

Gall-Peters is used extensively by the UN and would work as well.

Wikipedia has nice Tissot distortion diagrams for all three.

I would politely disagree with just this statement in the first answer

"Viewing anything on a global scale suits geographic projection, WGS84 being one of them."

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