A portable compass is the term used for a magnetic compass, compact enough that it can be used with one hand. This device has a sighting, which allows it to record a precise azimuth or bearing, used to find a location or a given target. These compasses come with instruments like lensatic or direct sights, prismatic sights and a simple type of alignment called notch and post. Since the sighting arrangement offers extra precision, a sighting compass will have better accuracy when it comes to measuring the objective bearings.
The portable compass term can be used by surveying and forestry type professions and in these cases their name are cruiser or forester compass. A portable compass might also come with geologist or surveyor transit pocket versions.
Portable compasses have been around for the last few centuries, at least the versions that had mechanical sighting. The first model to be used with just one hand, which had a sighting device included, appeared in 1885. In time, these compasses became the models more specialized like the Brunton Pocket Transit, which was invented in 1894. These portable compasses were used soon enough by practitioners of jobs like land surveying, cartography, archaeology, geology or forestry.
In the US, the portable compass was quite popular among the foresters which looked for a compass, so they could estimate and plot the timber stands. They could use a Pocket Transit for it, but it was quite expensive. That’s why they introduced a new type of portable compass, which was the cruiser compass. The traditional cruiser compass had a dry needle, a sighting notch, a large dial with individual degrees and adjustable declination. It also had a screw base, used for a monopod or a tripod.
Most foresters started using in the 60’s a compass design that was modern, which was liquid damped. That included models that had mirror sight protractors, like the Suunto MC-1 or the Silva Type 15. They were quite accurate for use in forestry, especially for use in a straight cruise line. Foresters, ornithologists, archaeologists, speleologists and geologists used models with direct reading for their survey work that needed precision. Some of these models were the Bruton Pocket Transit, the Suunto KB-77 (a prismatic compass) or the Suunto KB-14. Instead of using an azimuthal system (between 0 and 360 degrees), many of these models had an optional quadrant.
A portable compass, combined with maps and aerial photographs, will allow anyone to find out their exact location on a field, find out the direction which should be taken to reach destinations or landmarks, estimate areas and distances, look for points of interest. For extra accuracy, using professional portable compasses is still done with tripod mounts. Though these are still used, GPS receivers have started to replace them in recent years.
Marine portable compass
The marine portable compass is a device used by many sailors since the 20’s, so they could keep their ship on its course. They could also use it to record the bearings to the shore landmarks, so they could find out what their position was.
Instead of using a magnetized disc or needle, these marine portable compasses have a floating card design and liquid damping. This compass has a viewing prism and it allows the user to instantly read it. Plastimo Iris 50 and Suuno KB-77 and KB-14 are some examples of marine compasses.