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I cannot find any supporting documentation explaining why to use QGIS 64 bit instead of 32 bit or vice versa on www.qgis.org.

Is there any available?

I believe 64 bit should allow you use more machine memory than 32 bit, which may be useful for large files/datasets.

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    wouldn't it depend on if they had a 32 bit or 64 bit machine? – Ian Turton Jun 1 '17 at 8:41
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    Re iant's comment can you edit your question to include the specification of your hardware please (and then I think you'll find you answer your own question)? – MappaGnosis Jun 1 '17 at 9:44
  • @MappaGnosis Yes, I see what you mean, but I think that would be too specific. I thought the answer by Matthias Kuhn covers it without specification of hardware? I considered specifying it for windows users but that excludes a number of OS versions. – user97839 Jun 1 '17 at 9:48
  • If you have a 32 bit CPU or a 32 bit operating system (OS) you don't have a choice. In case of 64 bit CPU and OS you actually do have a choice and it's a valid question to ask yourself. – Matthias Kuhn Jun 1 '17 at 12:09
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As you correctly state, the 64 bit version allows for using more memory than the 32 bit version. One example where this is important is high resolution printing, there are real world examples of crashes with huge printings which are resolved by using the 64 bit version.

Compared to that, the 64 bit version has not really any downsides - apart from the fact that it only works on 64 bit machines. It might have a slightly increased memory usage but I have yet to find a scenario where this would even come close to outweigh its advantages. So while it's technically possible to use a 32 bit QGIS on a 64 bit operating system, there is no advantage in doing so (unless you require a 32 bit only driver like ms access, thanks @Techie_Gus). On the other hand, on a 32 bit operating system, a 64 bit QGIS cannot be started.

TL;DR

  • If you have a 64 bit machine and operating system, go for the 64 bit version (hint: if your computer is not very old, it probably is a 64 bit system)
  • If you do not have a 64 bit machine and operating system go for the 32 bit version (or get a new operating system and/or machine)
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If you need to work with Access databases within QGIS, you're bound to work with the 32 bit version as there is no 64 bit ODBC driver for Access. The good thing is that you can have both 32 and 64 bit versions of QGIS running alongside each other on the same machine, provided it's a 64 bit, of course.

  • This sounds like something that can be mission critical in certain scenarios. Is there a source for this or is this based on personal experience? – Matthias Kuhn Jun 1 '17 at 12:07
  • @MatthiasKuhn It's based on personal experience and some (probably a lot of) digging around as I'm currently working on a project involving data in an Access accdb database linked to spatial data. (On a side note nice profile pic. Glad to know there is a fellow GISer / climber!) – Techie_Gus Jun 1 '17 at 12:15
  • Actually there is a 64bit Access driver for ODBC the issue is a 64Bit Driver cannot be installed along side a 32Bit driver without a work around that I have not been successful with. So when I setup QGIS on machines here I check to see if they have a 32bit or 64bit MS Office/Access installed and install the corresponding QGIS otherwise I cannot link to Access tables for joins and relates. – Tyler Veinot Mar 22 at 15:08
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If you're trying to export large, high resolution compositions, then prefer using a 64 bit build. Otherwise the memory limitations for 32 bit builds can be restrictive.

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