Android GPS app to get your location on a paper map

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Android GPS app to get your location on a paper map

Postby arkolos » Wed 09 Dec, 2020 8:21 pm

Hello,

I’m presenting an Android app I’ve made that may be useful for some people in this forum: You Are Here. It allows to see your GPS location on any image / photo of map.

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You need to load the image / take a picture of the map in the app, then indicate the location of 2 points of the map and, from this, the app can show your current position on the map.

According to how the picture of map is taken and how it is positioned in the app, precision may be less important than in other tools. But, I still found it useful to follow a walk itinerary printed in a book for example. I haven’t tested it in the bush but I guess it may still be useful.

If you are interested, you can find the app link below. I’m open to comments/suggestions to improve it. It’s only available on Android for now but an iPhone app should come in the following weeks.

https://play.google.com/store/apps/details?id=com.you_are_here&referrer=b
Last edited by arkolos on Mon 14 Dec, 2020 8:39 pm, edited 1 time in total.
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Re: Android GPS app to get your location on a paper map

Postby Mark F » Thu 10 Dec, 2020 9:43 am

What I gather you are offering is a way to geo reference an image using 2 known points and then display the position on that image using the phone's gps. It will inevitably be inaccurate due to the inaccuracies created by the angle the photo is taken and lens distortion. You would actually need several points and then do a ortho adjustment of the image. For most of eastern Australia we can download properly georeferenced topographic maps from our state mapping agencies and see our position accurately shown using an app like Avenza.
"Perfection is attained not when there is nothing more to add, but when there is nothing more to remove".
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Re: Android GPS app to get your location on a paper map

Postby Son of a Beach » Thu 10 Dec, 2020 10:08 am

Good job. I used to do a similar thing in an old iOS app I used to sell. Yes, the accuracy is not perfect, but I found it to be accurate enough for some navigational purposes. It was certainly useful for the kind of tourist maps that you can pick up in brochures, etc.

Here's an idea that I also incorporated into my old app that you may like to try:

As an alternative to using two points for georeferencing, I also provided another method whereby the iOS standard map could be overlaid with the scanned-in map with whichever one of them is on top being semi-transparent. Then the user can zoom/pan the two maps around until they fit each other exactly (or as close as possible).

If done well, this is quite intuitive and can be much easier than trying to figure out precise coordinates for specific locations.
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Re: Android GPS app to get your location on a paper map

Postby arkolos » Thu 10 Dec, 2020 6:36 pm

Hello,

Thank you for your messages.

Mark F wrote:What I gather you are offering is a way to geo reference an image using 2 known points and then display the position on that image using the phone's gps. It will inevitably be inaccurate due to the inaccuracies created by the angle the photo is taken and lens distortion.

Exactly. Indeed it may not be exact but when I tried it it was still accurate enough to help to follow an itinerary. The issue I often had using non geo referenced map is that I sometime see on the map I should turn at one point but I don't know when I reach this point.

For most of eastern Australia we can download properly georeferenced topographic maps from our state mapping agencies and see our position accurately shown using an app like Avenza.

Thank you, I didn't know about this app! Note that even if you already have a georeferenced topographic map , You Are Here can still be useful to import a map of your custom itinerary

map could be overlaid with the scanned-in map with whichever one of them is on top being semi-transparent

Interesting that you also made an app about it! Thank you for the suggestion. There is already something similar in You Are Here. After you've chosen 2 points on your map and found them back on a standard map, you can edit the placement the app suggest by moving a transparent standard map onto it

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Re: Android GPS app to get your location on a paper map

Postby ribuck » Fri 11 Dec, 2020 12:04 am

I have used the Android app "Custom Maps" which does the same thing. The Tjoritja NP maps are available for free as high-quality scans rather than as electronically-generated files. By georeferencing them with the app, I get accuracy better than 100 metres on a 1:100000 map.
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Re: Android GPS app to get your location on a paper map

Postby Warin » Fri 11 Dec, 2020 8:16 am

In order to accurately geo reference a map you need more than 2 points.

2 points would be enough if you have the datum and projection information.
Without the datum and projection information a vast number of pints would be required...

The larger the area covered by the map the larger the probably errors will be.

Suggest you do some reading.
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Re: Android GPS app to get your location on a paper map

Postby neilmny » Fri 11 Dec, 2020 9:17 am

Warin wrote:.......... a vast number of pints would be required...


Freudian slip perhaps :lol:
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Re: Android GPS app to get your location on a paper map

Postby MrWalker » Fri 11 Dec, 2020 9:03 pm

Warin wrote:In order to accurately geo reference a map you need more than 2 points.

2 points would be enough if you have the datum and projection information.
Without the datum and projection information a vast number of pints would be required...

The larger the area covered by the map the larger the probably errors will be.

Suggest you do some reading.

Are you sure you need a vast number more? The method does appear to work.
I've done something vaguely similar with state maps just using two points to lock in every point on the map (or close enough for all pratical purposes). This was without having any other information about the scale of the map, although they were probably all similar projections (the north/south state borders were vertical lines).
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Re: Android GPS app to get your location on a paper map

Postby arkolos » Sat 12 Dec, 2020 6:08 am

Warin wrote:In order to accurately geo reference a map you need more than 2 points.

2 points would be enough if you have the datum and projection information.
Without the datum and projection information a vast number of pints would be required...


I assumed that when we imported a map in the app it was in projection EPSG:3857 . I agree it may not always be the projection of the all the maps but the result seemed ok for the maps I tested with. And I think most of the maps imported in the app will be covering small areas so the projection used would not change much.
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