Old maps are my favourite tool for investigating Hong Kong's history. Sometimes they help answer questions, eg finding which building used to stand at a certain place in Hong Kong.
Other times the maps raise their own questions - I'll find something marked that I didn't know about before, and that gets me interested to learn more.
So where to see these maps?
Here on the Gwulo website
Gwulo provides an online map that shows the locations of buildings and other places we've documented, with the option to see them displayed on a variety of modern and historic maps. Here's the tutorial explaining how to use the maps on Gwulo.
We also have a selection of scanned maps of Hong Kong to view.
The website http://www.hkmaps.hk/ has an excellent collection of old Hong Kong maps, viewable online as a scanned image, or overlaid on a modern map. It also has a good list of other places you can find old Hong Kong maps.
[As of 2017 This book is out of print and no longer available new, but used copies are available, eg on Amazon.com] The easiest way is to get a copy of "Mapping Hong Kong - A Historical Atlas", by Hal Empson. It has old street maps, topographic maps, as well as some aerial photgraphs. It's a big book and well printed, so there's lots of detail to see. It's printed by the HK Government Printer, so you won't find it on sites like Amazon. Instead you can buy it online through the government bookstore, or visit one of the Survey & Mapping Office's sales outlets (call first, to check if they have it in stock). At the time of writing, the book costs HK$498.
If you'd like to take a look at the Mapping Hong Kong book before you buy, there's a copy in the Map Library, 5/F Central Library. The Map Library also has a good range of large-scale old street maps available to look at, and map-reading tables so you can read in comfort. I struggled on my first visit to find the exact map I wanted (you have to submit a request to the librarian, and they bring it out for you to look at). The trick is to ask for the "Hong Kong Map Catalogue" folders. There are two, one for recent maps, and one for old maps. For each map series that is available you can see the dates and areas covered. From there you can point out the individual sheets you need.
The other good source is the collection belonging to the Survey & Mapping Office (SMO). See "Historic maps at the Survey & Mapping Office (SMO)" for details.
Old maps online
I haven't found any good online access to these old maps.
The Public Records Office search page lets you search for maps, but... most results don't have any images attached, and the ones that do are too small to be of any use. So, expect to need a visit to the PRO building if you find anything interesting.
The Public Library search page (remember, only works in Internet Explorer) does return maps that you can zoom in to and see the detail. But I've struggled to make the search engine return what I'm looking for. (Let me know if you've cracked it).
The simplest way I've found to access the Public Library's map collection is to go in via the government's '18 Districts' website. It shows you a list of selected maps for each of district in Hong Kong. Here's the map page for 'Central and Western' district. On the top right of that page you can click to select any of the other districts.
I often look at the modern online maps too, especially when I'm looking for remaining signs of physical features like fortifications and tunnels. You can often see their shape if you zoom in to the map at the highest magnification.
There are several maps that are based on the government's surveys. The commercial sites (YPMap and Centamap) are fastest and easiest to use. There are also several government sites that display the maps, eg the Statutory Planning Dept.
What am I missing?
What other resources are there (online or offline) to view old maps of Hong Kong? And any other tips on using the tools listed above?