Suppose a team of explorers touched down on an Earth-like planet which do not generate its own magnetic field and there are always a thick layer of cloud covering the sky at all time. My question is how can they create an accurate map in such a scenario? (note there are no GPS satellites here!)
Find north with a gyrocompass. Map with SLAM drones.
A gyrocompass is a non-magnetic compass which is based on an internal fast-spinning disc and rotation of the Earth to find true north. This is not the same thing as a gyroscope. A gyroscope alone is not sufficient for long term marine navigation.
The Wikipedia page lists two advantages.
Gyrocompasses are widely used for navigation on ships, because they have two significant advantages over magnetic compasses:
- they find true north as determined by Earth's rotation, which is different from, and navigationally more useful than, magnetic north,
- they are unaffected by ferromagnetic materials, such as ship's steel hull, which change the magnetic field
More advanced versions are the HRG gyrocompass and fiber optic gyrocompass. Which are based on a hemispherical resonator gyroscope and a fiber optic gyroscope respectively, both solid state devices, allowing for a maintenance free instrument.
Mapping is a slightly different story. The explorers can use drone fleets with inertial navigation and/or ultra wideband positioning to photograph and map the surrounding areas. This technique is called SLAM.
This is, of course, assuming you don't deploy GPS satellites in orbit, because those do work through clouds.
A sufficiently accurate gyroscope can be used to find true north to some precision by determining the angular rate of change of the unit (The military uses such north-finders for artillery batteries).
Using a polarizing filter, the location of the sun can be determined to within several degrees. While this is hardly precision navigation, it's certainly better than nothing, as it allows a rough determination of both latitude and longitude.
Of course, a "team of explorers" is not about to create an accurate map of the planet when the planet is always overcast. With no satellite photographs, and large-scale aerial photography unavailable, only local maps can be produced the old-fashioned way, by surveying teams.
If the overcast is high enough, some aerial photography may possible, and the science team could set up a series of radio transmitters to provide what is essentially LORAN navigation for the mapping aircraft.
Stars emit in a much broader spectrum of wavelengths than just light. You can use a simple radio telescope to locate the sun even through the thickest of clouds. This would allow you to pin point your latitude. Given you have space travel, some one should have an accurate watch so you could find your longitude quite simply too.
Once you have worked out one point you can start surveying the surrounding area. We used to use trig points for this. This allowed us to make reasonably accurate maps.
Then when you want to navigate you can use the hills as reference points, as well as working out your own latitude and longitude with the tiny radio telescope
Okay, first, colonists coming down from space totally would set up a satellite navigation system. Without any launching costs from getting the satellites surface to orbit it would be too trivial not to do.
But given the premise, Oldcat is actually correct with surface emitters. Before GPS ships used a location system based on radio beacons. And radio navigation seems to be still in current use.
So simply set up some radio beacons sending a time signal around your base. Add some at other important locations. As long as the clocks in beacons work and you know the relative distances of the beacons, it is fairly simple to calculate location.
The area covered by the system can be extended gradually by adding more beacons. This is not particularly good system for global navigation, but it will cover all the area you actually use. And will allow you to always know the direction home.
The same way maps have always been made: using large landmarks. Presumably the planet is Earth-Like enough to have large mountains, forests, oceans, rivers, lakes, and potentially even cities and roadways. For navigational purposes, and to map the general shape of the world, basing your maps off of landmarks would be perfectly acceptable.
Piecing together a full global picture would then simply be a matter of taking a globe, and projecting the shape of those drawn landmasses onto it as accurately as possible - the 'top' of the globe would be chosen abstractly, but that's already the case for Earth. On this alien planet, it'd probably just be the highest peak (Measured by distance and declination) or divided in half by landmass.
A high resolution, low drift inertial navigation system (INS) could give them position information which would form the basis of a map. Mark the landing zone as 0,0,0 and move off from there. Combat drift by resetting the INS whenever they return to base. This requires no preloading of coordinates from space and operates anywhere the laws of physics apply. As an added bonus, since INS keeps track of accelerations, you catch not only x and y positions, but also altitude measurements.