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It's after the apocalypse. Most modern technology has been destroyed, either by disaster or neglect.

How could you locate a particular set of GPS coordinates without the aid of the satellites or modern technology?

Question Breakdown:

  • What equipment or knowledge would you need?
  • How hard would it be, and how precisely could you determine the location?

Thoughts:
My first thoughts are that you would either need an old-world map with the coordinates, or you locate a landmark with known coordinates and triangulate from there

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    $\begingroup$ Best case, find an old map with the GPS coordinate grid and find (major) landmarks. Rivers might move, mountains won't. Alternatively, translate the GPS into longitude and latitude and use old-fashoned stellar navigation. (This requires a decent watch.) $\endgroup$ – o.m. Jul 26 '18 at 4:37
  • $\begingroup$ Have edited to focus entirely on process instead of character and situation and voted to repon $\endgroup$ – Chromane Jul 26 '18 at 4:54
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    $\begingroup$ @o.m Good idea about the watch - I like the idea of using a sextant to try and find the coordinates. Your point about landmarks is well taken too $\endgroup$ – Chromane Jul 26 '18 at 4:54
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    $\begingroup$ +1 @o.m. Swiss watches are 17th century tech (possibly as early as 14th century, but I'm going for something that is recognizably a "decent" watch) $\endgroup$ – pojo-guy Jul 26 '18 at 5:00
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    $\begingroup$ I apologize, but I'm going to downvote the question for lack of research. A sextant and a clock were plenty accurate from the 1700s until the 1960s. Search Google for "pre-GPS navigation". $\endgroup$ – JBH Jul 26 '18 at 7:13
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It depends on your world, but you've already stated the first couple of options to try...

Basic items you'll need no matter what

  • Decent Accurate Watch (something that might be hard to find in the apocalyse even harder to find one that is accurate compared to the pre-apocalypse)
  • A Compass, preferably an orienteering compass
  • A very good quality, high resolution map of the country your reference material is in
  • Some string and Pins

Then... you'd need to head to a library, in there brush up and pretty much all of navigation from the history of man. i know that seems a bit over the top, but it's how humans learnt, always expanding on previous knowledge, so this would really take a while to learn

Triangulation

This is incredibly complicated and i'm not going to copy and paste a huge amount of text from another site to explain how to do this. so check out this site,

http://www.compassdude.com/compass-triangulation.php

It explains it well, with pictures to guide you. but instead of things you can see, you taking items on a map you know the locations and coordinates of to aid you, however I invite anyone to try my below option, see if people can do this without using GPS or google etc.

  • The White House in Washington DC is at 38.8977° N, 77.0365° W
  • The Statue of Liberty is at 40.6892° N, 74.0445° W
  • Alcatraz Island is at 37.8270° N, 122.4230° W

I've picked these starting points as I remember learning about GPS from a textbook way back when, and the three points had there co-ordinates stated in the textbook, so you'd probably be able to get them in the apocalypse

  • [Easy Mode] Now find which building is at 38.8899° N, 77.0091° W
  • [Hard Mode] Now find which building is at 51.5007° N, 0.1246° W

If you've managed easy mode then fair play to you, its incredibly difficult, because you probably quickly find out that try to pick a single small (comparatively) location even when close to one of the original points is very hard, and that's using a map of just the US which you can at least get on a fine enough scale to do this, as soon as you want to go further afield (Hard Mode) it becomes infinitely more difficult, the greater the distance the rougher the maps then the lower chance of finding the object.

Anyone who managed Hard Mode... sorry but i think you may have cheated... if you haven't then wow, but chances are you either cheated or you worked out buildings in that rough area and then guessed., please prove me wrong.

Translate into Longitude and Latitude

This has a higher chance of working, a library would have books that would explain how to convert it, or at least convert it the other way, and therefore just reverse the process. the bizarre thing is after about half an hour of googling, i only found a single link to how to do it yourself rather than a link to a converter

  • The whole units of degrees will remain the same (i.e. in 121.135° longitude, start with 121°).
  • Multiply the decimal by 60 (i.e. .135 * 60 = 8.1).
  • The whole number becomes the minutes (8').
  • Take the remaining decimal and multiply by 60. (i.e. .1 * 60 = 6).
  • The resulting number becomes the seconds (6?). Seconds can remain as a decimal.
  • Take your three sets of numbers and put them together, using the symbols for degrees (°), minutes (‘), and seconds (“) (i.e. 121°8’6” longitude)

I copied this in as it wasn't a huge amount of information, i got it from here:

https://exain.wordpress.com/2008/07/17/doing-gps-conversion-degrees-to-latitude-longitude-and-vice-versa/

From there get a sexton and brush up on your stars, lets just assume you could find a quality sexton in an antiques store, then follow the stars. again i prefer not going to copy and paste someone else's work when its more than a few lines

https://www.pbs.org/wgbh/nova/shackleton/navigate/escapenav.html

This would take extreme dedication on the characters part to learn all of this, it would take a long long time, but most Apocalyptic RPGs, TV series, and novels do have characters that either do exactly this, or something that is equal in learning. so i think its likely that if portrayed well then it would work.

My suggestion would be to translate the coordinates to know where you are going, and then also translate the points of reference, and then triangulate you position when in sight of your target area to check you are where you think you are.

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  • $\begingroup$ I would also add that the (multiply) by 60 give you only coordinates by smallest unit which is second and that is 30 metres (in latitude) In longitude you would need to use "1° of longitude = 40075 km * cos( latitude ) / 360". And recalculating GPS to minutes and second can be off by 2-3 seconds. So the accuracy is around 100 metres. $\endgroup$ – SZCZERZO KŁY Jul 26 '18 at 9:25
  • $\begingroup$ Since building are large, getting the coordinate of White House won't give great accuracy. Better searching for topographic strongholds, which are point size and give better accuracy. $\endgroup$ – L.Dutch Jul 26 '18 at 9:34
  • $\begingroup$ @SZCZERZOKŁY, fair point well made, basics are still the same though $\endgroup$ – Blade Wraith Jul 26 '18 at 9:42
  • $\begingroup$ @L.Dutch i agree, but i choose those as I've seen them referenced in textbooks, i don't ever remember topographical points in text, and in the post apocalypse, we'd only have written text to go by. I've seen Everest in a textbook as well but figured map scaling would make that fairly useless. $\endgroup$ – Blade Wraith Jul 26 '18 at 9:43
  • $\begingroup$ Statue of Liberty is a good landmark - she seems to partially survive any fictional apocalyptic scenario to date $\endgroup$ – bukwyrm Oct 24 '18 at 11:12
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Many cities already have plaques with the meassurements. Look for those survey markers.

In Uruguay these are made in granite with bronze labels. In other countries there are metal disks. Survey marker

survey markers

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    $\begingroup$ Not just cities. In the western US, you often find such survey markers on mountain peaks and other landmarks. $\endgroup$ – jamesqf Jul 26 '18 at 17:28
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This depends on the planet. The technique could be different depending on the world you have in mind, as they could vary in magnetic poles, true-north poles, daily rotation, etc.. My answer, like the others here, assumes Earth, or a very Earth-like world.

GPS coordinates generally come as a pair of numbers. One number, the latitude, tells how far you are between the north and south poles from negative 90 degrees to positive 90 degrees (or 90S to 90N). The other number, the longitude, tells where you are in the other direction from -180 to +180 (180W to 180E), where around the circumference of the world you are.

Both of these numbers are generally given in 1 of 2 forms: DMS (Degrees-minutes-seconds), or as a decimal number. The latter is simple: 3.25 degrees north latitude is just what it sounds like, a quarter of the way between 3 and 4. DMS divides it up into smaller units, where there are 60 minutes in a degree and 60 seconds in a minute, so 3.25N equals 3 degrees, 15 minutes, 0 seconds N.

Latitude and longitude can be measured without any need for any map or compass. Sure, having these tools would be nice and allows you to take short cuts, but it is not too difficult to re-invent this wheel when necessary. However, you must measure latitude and longitude separately, very differently from each other, and even though it's not difficult conceptually to understand longitude measuring you do need an accurate clock.

Latitude

There are multiple ways to measure latitude. I will focus on the stars since they are easier to remember and the most reliable as long as the night is clear.

In the northern hemisphere, measuring latitude is as simple as measuring the angle to Polaris, the north star. Straight overhead means you are at the north pole (90 degrees north latitude), and straight out on the horizon means you are just above the equator (nearly 0 degrees latitude). Between the two, you just measure the angle of the north star in the sky, and that literally and directly tells you your latitude. North star 30 degrees up? You're at 30N. 50 degrees? You're at 50N. In fact, you can roughly estimate your latitude just by eyesight with no tools at all. Using a quadrant, sextant, or similar device will help you measure more accurately and finely.

You can literally use a protractor for this, and hang a string from a weight to help you estimate what is straight up/down to help measure against. You can also create a quadrant out of paper by cutting out a quarter-circle and measuring off the degrees on it and sighting along the edge.

How to Find the North Star

In the southern hemisphere, the process would be similar, except that there is not a bright star directly above the south pole, unfortunately. Instead, you need to estimate the south pole point in the sky and measure against that. You can estimate it by using the "southern cross".

How to Find the South Celestial Pole

When close to the equator it is a little bit different. I think you might be able to use the constellation Orion for that. I will look into that and get back to you for that.

Longitude

To measure longitude, you have to pick a spot around the earth's circumference to call 0 degrees. Right now, the standard is that 0 degrees longitude, called the "prime meridian", is the line between the north and south poles which passes through the royal observatory in Greenwich.

To calculate longitude, you keep track of the time at the prime meridian, and you calculate the difference between that time and your own, generally at a well defined time such as noon.

There are 24 hours in a day as the sun goes overhead around the entire planet (due to the planet rotating). You can use basic algebra to equate those 24 hours into 360 degrees of a circle around the planet. So each hour difference equals (360/24) = 15 degrees.

Example:

So you have a pocket watch whose time is set to the local time zone at the prime meridian. You want to know your longitude, so you measure shadows to figure out when the sun is at the very highest point in its arc over your head for the day, as that is the exact moment of noon at your location. Noon at your location coincides precisely with 10:30 on the watch, so there is a difference of 1.5 hours. At 15 degrees per hour, that means that your longitude is (15*1.5) = 22.5 degrees, or 22 degrees 30 minutes of longitude.

Hopefully you still have a watch around. If not, you might have to measure latitude only.

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If you have a point with known coordinates, then any other point on the face of the planet can be found using sufficiently accurate gyroscopic means.

As you travel, your exact speed and heading (maybe bearing) are known. This technology is used on submarines and has become surprisingly accurate. It is also used in missiles to defeat GPS jamming technology.

While there is error associated with these systems, it normally comes from things such as rocking of a boat and very high speeds. At low enough speeds and decent handling of the device, you get outstanding accuracy.

Another possibility is a counter-rotating optical gyroscope. These do exist and can give compass bearings with amazing accuracy. If you know the bearing information, your heading is simply just following that line until you reach your point. Measuring distance incredibly accurately is actually relatively simple.

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