Accurate mapping of public indoor spaces?

This forum seems pretty morbid, but maybe someone follows and can help. . .

All the mapping I’ve done has been outdoor and for there a decent set of GPS tracks, GPS tagged photos and/or aerial imagery goes a long way toward allowing one map things.

Now that I am looking into mapping of some indoor features I am at a loss on how, in the general case, to accurately locate things indoors. The examples I’ve seen all seem to start with a fairly accurate floor plan and then, by survey, locate items based on that floor plan.

But what does one do if you don’t have a floor plan that is at least roughly to scale? I am thinking of publicly accessible area (stores, railway stations, etc.).

That’s when the fun begins…

Of course it’s hard to be perfectly accurate when you don’t have a map. For an example, you may have a look at the Châtelet metro station in Paris, which I have recently finished mapping (now I should do the Halles underground mall) :

Not only I had no map, but it’s an old underground metro station with very complicated corridors, small stairs and very few right angles…

So I started from the entrances, walked along the corridors and mapped very slowly. I guessed that many corridors and metro lines, which are more than 100 years old, were built under the streets (except for the more recent ones which I knew were built with a drilling machine). And I validated and fixed my guesses when connecting one entrance with the other through a maze of corridors.

So of course there might be some location mistakes, but I don’t think they exceed 5 or 10 metres.

Very impressive. I’d be happy to figure out a way to map the inside of a single level shopping center without looking too conspicuous.

Maybe my best choice is to practice estimating distances visually and by counting foot steps.

To look completely unsuspicious, just wear an orange warning west and a clipboard …

You may see and walk through the Châtelet Metro in 3D (Most impressing in Google Cardboard):
Use PageUp/Down or +/# to elevate/dive
OSM-go is not well done vor indoor yet. If some of you like to join, we could do it as a team.

Abowe and below:

I’ve mapped several train and subway stations across Germany and Austria.

My approach is to first select a starting point from an aerial image - like a stair case leading to an underground area. From there I count the steps of the stair well and then walk along the walls of the underground corridor counting my steps. This is a good enough method to measure distance for rough mapping purposes. Simultaniously I draw a sketch of the mapped area and the measured distances. When the wall changes direction and the angle is not rectangular I usually guess the angle. But in critical situations I also use a simple angle measuring tool. At important points like staircases leading to an additional underground level I take control measurements, through walking and counting steps to other well known points. When you reach an exit, which is identifiable from an aerial image, you can check the accuracy of your measurements.

This is a quite time-consuming method to map indoor areas, but the quality is good and there is no need for expensive devices or special expertise. My measurement errors rarely exceed 50cm.

We have used a laser measuring tool for this in buildings we need to check footage on. If you have access to the building, counting windows or other unique building landmarks and seeing how they align with outside items, like a parking lot cut-out, stairs, a sign, etc . . .
But I agree with above poster, start the map with item that goes through the whole building, like an elevator, staircase, etc.