By Joe Mehaffey - from "Automatic
Address to Address Routing GPS Automobile Navigators What to
expect from the currently available equipment" rev. 07/10/02
"Maps: The maps required for a GPS automatic navigator which can
design you a route from address A to address B are highly specialized.
In addition to the need for highly accurate maps, the map database must
contain a large amount of data about highway and roads. This road
information includes average travel speed classifications, one way
streets, names and road numbers of all streets, expressway ramp
details, and dozens of other programming details.
One of the main disappointments
of some people is that the maps are NOT PERFECTLY ACCURATE or they
find their favorite street in a subdivision is missing. Forget it.
None of the currently available maps are perfectly accurate. And they
do not have to be perfectly accurate for the user to receive quite
satisfactory results. For instance, roads and highways are being
built, moved, and renamed and renumbered all the time. Many
municipalities are lax in getting their changed map data to the proper
agencies and are in arrears for YEARS. Generally speaking, maps
appear to be from 2 to 4 (or even more) years old. Some roads (even
Interstate Highways) can be out of position by hundreds of feet in
spots. (So many roads, so few map makers.) As a result, you cannot
pick up a single printed or electronic map that is completely
accurate. Some ARE better than others. NavTech and MetroGuide II
maps are among the best. BUT! Don't expect perfection! You won't
NOTE: The above comments apply
to the REALLY GOOD quality maps by NavTech and Etak (note - I think
he's referring to TeleAtlas who purchased Etak) and a few others
as used in Car Navigation systems. The "not so good" quality maps
(such as Garmin R&R, Lowrance Streets, and Magellan MapSend Streets,
the WorldMap offerings and basemaps in handheld units) can have much
larger errors. I have seen roads displaced by almost half a mile in a
few spots. Sometimes the shoreline on basemaps is displaced so you
seem to be driving in the water. Forget it! Things are improving
every day, but perfection is quite a ways off"
Map source, "snap to road" and naming
want to mention a couple of points that you may or may not know about
navigable digital maps.
Tiger data is the starting point for NavTech and TravRoute's digital maps.
The positional accuracy is in the neighborhood of 6-10 meters. It used to be
To display a vehicle Icon on a road segment the developer must make the
decision to snap it to the road line. What do you do when the vehicle is not
on the road, say in a large parking lot? In addition the combined
interaction of the maps positional accuracy and GPS error can often fall
outside the developers criteria for snapping to a road line.
Road segments typically have 3 name fields. One of the fields is supposed to
be the locally preferred name but, displaying this field is not always the
best choice. The I40/CORONADO HWY example you used is a case where the
developer is displaying 2 of the field names. You will probably find that as
you drive I40 the secondary name changes with each new municipality.
Unfortunately, the three name fields and their level of importance is the
only information the developer has to draw from. And, to further exacerbate
the issue sometimes there are no names."- posted by SteveT on the
PocketPCPassion message board
By PanEuropean in the
"Database Accuracy: Garmin,
unfortunately, is at the mercy of the people (usually governments) who
provide the _vector_ map data. I know from extensive riding experience
that the map data for Switzerland, Germany, Belgium and Luxembourg is
absolutely fantastic - every single street, path, and cart track in those
countries are listed, with great accuracy. The only problems that
exist in those (highly developed) countries are 'typographical' - meaning,
by example, I have found a few places where
pedestrian walkway was coded to be a road suitable for vehicles, etc.
Get down south, into Spain, Portugal and Italy, and the quality of the map
data falls way down. Heck, in Spain, big 4 lane autopistas that have
been open for over a year are not on the database, compared to Switzerland,
where little roundabouts in residential neighborhoods that were built in
spring 2002 ARE in the database. In Lisbon, all the streets in the
city are listed - but the fact that half of them are one way is not listed.
There's no way that NavTech surveyed these Swiss residential neighborhoods
in the 2 months between roundabout construction and release of the European
support auto routing - that information must have been supplied to NavTech
by the Swiss Federal Mapping Office, most likely in advance of the actual
construction of the roundabout.
AFAIK, Garmin buys the cartographic data from NavTech, and NavTech starts
their data collection process off by buying _vector_ street and road data
from the federal governments of the various European countries.
NavTech then sends their vehicles out to drive the main roads and collect
additional data, such as signage, speed limits, weight restrictions, etc. to
refine the information even further. I don't think NavTech has got
around to doing a lot of their own data collection work in the Southern
European countries yet. There will be an update released for CN Europe
this fall, that will help some and will correct the gross errors, but really
high quality, complete coverage for the Southern European countries will
depend, for the most part, on the federal governments of these countries
compiling accurate and complete _vector_ data describing their roadways.
Don't bet on this happening overnight - 5 years is my personal guess, and
only if they get EC funding to do it."