why is my emap not accurate?

Howdy timewaster,

Well, this is the second post today that I've read that I have talked to someone about over the weekend. We were heading out to Sandy Creek on the weekend to check out some camping spots when I noticed that, even though the road we were on was marked on the GPS, our track taken was off the road and heading away. I also have a Garmin Emap, I've never driven in the ocean as yet, but it doesn't seem too accurate. Whilst I know that they are 5-20m inaccurate, you'd suspect that it should keep you reasonably on track. Be interested to hear the comments.

Cheers

Chris.

Which mapping are you talking about ?

If you are talking about the basemap on the eMap then errors of up to 1 km are not uncommon. This is very old data and only supposed to give you an overview.

If you are talking about Mapsource Metroguide Australia then this is much more accurate.
It can still have errors; 50 metres in the city is not uncommon and occasionally 2-300 metres in some rural areas.

Inaccuracy could be due to a number of things
1. Bad satelite signals - generally only temporary though if you in thick forest you may not be able to get enough statelites to get a good position reading ever. You can generally spot this happening if you are stationary and the position readout changes radically as satelites come and go.
2. Inappropriate map - some maps are made for using only zoomed out and not totally zoomed in eg basemaps - not sure what the story is with Garmins - but with magellan basemap - at close up zooms you will see that the coastline is only a rough angular shape (less points - to reduce file size) If you want best accuracy you must use detailed maps (DiscoverAus for Magellans). Basically dont expect basemaps to be accurate with respect to linear features - coastlines and roads etc. when zoomed in.
3. Just a bad map - what are Garmin maps like?

Re entering in towns
Wouldnt think the towns would be too badly placed on GPS map - are you entering coords in the correct format? Some confusion can be had with decimal degrees minutes and seconds and just plain degrees minutes and seconds. Double check. Also keep in mind the datum though this wouldnt be noticable if entering town locations as they are bigger that the error you would get (200m max).

Cheers
Greg

You should also consider the scale error that occurs.

What I mean is if you have zoomed in to say 100 meters you may find that your track is way off the road, If you measure the distance between where the GPS places you on your screen map and where the road is depicted you may find that you are shown to be 5, 10 or 20 or so meters of the depicted track.
This occurs because of the natural error as detailed in the above posts of the satellites and to some degree the inaccuracy of the map itself.

When you zoom out the scale increases and you find that you are now much closer or in fact on the road.
A 5 meter error just looks so much bigger on a 100 meters scale than a 2 km scale.
I don't find it a problem as Long as it follows the general direction and line of the road as depicted.

Like any map it is indicative only of where you are.

there are several different reference systems used over the years and WGS 84 is the cmmon one now though there is a newer one now. 20 or 30 year old survey maps and marine charts were found to have reference errors in them and often gps will allow you to enter a different reference which eliminates some of the error

The Americans own the satellites which run the system. During times of war etc (like now in Iraq), they play around with the accuracy so their enemy can't get accurate figures from their resources.
The troops on the ground plug the code into the GPS which counteracts this so they get accurate numbers. This has been going on for a few years. They keep the accuracy fairly close so us civilians still won't get lost though!
Nothing you can do about it. Just be aware of the above. It's not like it will ever be 100m etc.