Horizons Unlimited - The HUBB

Horizons Unlimited - The HUBB (https://www.horizonsunlimited.com/hubb/)
-   Navigation - Maps, Compass, GPS (https://www.horizonsunlimited.com/hubb/navigation-maps-compass-gps/)
-   -   Bowditch (https://www.horizonsunlimited.com/hubb/navigation-maps-compass-gps/bowditch-28621)

smitty 5 Aug 2007 17:34

Bowditch
 
I have noticed numerous errors in concept regarding usage of GPS, compasses, maps(charts), and applications of these regarding finding ones way out of "nowhere" ie "which is the best compass or GPS etc.". A "cheap" unit in many cases may assist you as well as an expensive one if it is properly used. In many cases it's not the "unit" that is at fault, but the transfer of the data to paper or the interpetation of the data. GPS is based on the concept that the earth is a "spheriod" and it's actually an "oblique spheriod". Transfer of data can conflict when you transpose from "electronic to paper". "How come I'm right at such and such location and the GPS says I'm a mile south?". "Something must be wrong". There is a publication "The American Practical Navigator" by Nathaniel Bowditch which was simply written, very understandable, and is found aboard Merchant and Naval ships worldwide. It explains compasses, GPS, and celestial science and their practical uses. It can be had on line, at any nautical bookstore, or at any "library". If you intend to make the big trip, it's got great information that, in essence, will build you confidence in "Gettin yourself around". Good luck, good reading, and good riding! Captain Smitty

Frank Warner 6 Aug 2007 01:29

A mile is rather large to atribute to different dataums and/or projections ..

I've experienced map errors .. of several miles. Unless I know the map maker by other maps then I'd rather not trust a single map, if two maps agree then they are probably right.

smitty 6 Aug 2007 07:30

Mapped Out!!!
 
The scale of the map, the type of projection it is based on, and the date of the survey used to make the map(some "new" maps are compiled with old data on parts of the map) can effect the accuracy of where you are. If you are using a small scale (large area map) and have a large scale map (which covers a particular area of the small scale map) and let's say you are on a particular road or area which crosses back on to the smaller scale map, the plotted positions will be usually be different. Let's say on the large scale map you seem not to be on the road, but slightly south of it by GPS plot. When you cross over to the small scale map on the same road, you may be 2 miles north of it by plot. This is due to conforming a flat projection (map) from a sphrical one (the earth). Usually, the map is most accurate at it's mid lattitude (the center of the map east to west) and loses accuracy as you go north or south. The amount of error you could say is not as relevent as knowing this occurs when reading maps. If all else fails, do what I do, stop for a beer and ask "Hey, where the hell am I?" Don't get lost...... Smitty


All times are GMT +1. The time now is 00:29.


vB.Sponsors