Dictionaries describe how words are used rather than prescribing what they mean. Meanings change, and the origin of a word is often only loosely related to its current meaning.
Navvies: "Navigate" comes from latin
navigare meaning
to travel in a ship.
In due course, this developed a wider meaning of
travelling by water, and then it developed to mean
any stretch of water that was suitable for, or modified for, travel by boat. Today, in the UK, a river licence for a boat covers those parts of the river that are designated a "navigation" and theoretically you are not meant to stray off those parts of the river into backwaters and side streams.
In the sense of
stretch of water suitable for a boat to travel on the word
navigation was used for canals designed for this purpose. The word
canal just means "channel" and might equally be used for an artificial channel used for drainage or irrigation.
So when people were working on the new navigations (
i.e. digging/building canals for transport purposes) they were referred to as
navigators and then as
navvies — just as a person who planes wood for a living is a
chippy and someone who deals with electricity is a
sparky.
I really don't think it's any more complicated than that. Certainly the illiterate, rough and ready labourers who were called
navvies were not the people who were doing the complex survey work and navigation to ensure that the new canal followed the contours.
The use of
navigate to refer to finding one's way around
by land using a map and compass is more recent.
Navi... is specifically linked to
navis (ship) and
navigare (travel by ship) and the English word
navy.
Pikey: This is now generally regarded as an offensive term for travellers and not one that I would use except in a discussion about the origin of the word itself.
@Trevp666 gave a good explanation of what a turnpike was: a toll road protected by a barrier.
Pike means
spike and is linked to words like
peak,
pickaxe, and German
pickelhaube (a spiked hat). I have read that the turn pike was a spiked barrier rather than a simple pikestaff.
I read many years ago that
pikey was derived from
turnpike sailor. On this analogy, the traveller or gypsy (not the same thing*) was likened to a
sailor because they were away travelling for most of the time and would return or pass through from time to time, and the
sailor was a fanciful reference to the canvas covers on their wagons.
I am sceptical about this. It sounds like an imaginative folk etymology. A far simpler explanation is that people who lived and travelled on the turnpike or pike (road) were called
pikeys.
*Gypsies/travellers: Two different groups, linked in the popular imagination y the fact that they both traditionally lead a semi-nomadic lifestyle. Gypsies are Romani/Romany people, but the Irish Travellers are a genetically and culturally distinct Irish group believed to have diverged from the settled community around the time of Cromwell, or possibly earlier.
Gypsy is also often seen as an offensive term, but the situation is made more complex when Tyson Fury (of Irish traveller stock) refers to himself as "the Gypsy King".