The Maps in my new (Garmin,  Lowrance, or Magellan) GPS are not perfectly accurate.  Why is that?
Updated November 2005 (Add Mexico Maps)

Depending on where in the world you are,  there are a number of different answers to this question.   Generally,  each manufacturer attempts to make his GPS system as competitive as possible in the marketplace.  As a result,  a good number of engineering and cost compromises are necessary during the design of the equipment.  One of these are in the area of maps.  Much of the specifics below relate to Garmin maps but the principles apply to all  consumer GPS vendor's maps.

1)     Base Maps:
            Base maps are not as accurate as more highly detailed maps.  In an effort to keep costs low,  each manufacturer purchases as low a cost basemap as possible while maintaining what they perceive as "adequate accuracy".  Generally,  the maps are chosen so as to add little or no added costs to the base price of the unit via map royalties or expensive one time costs.  As a result,  you should expect that the base maps may sometimes  be off in the range of 1000+ feet (300+ meters) or more in some spots.  Generally,  these base maps are ROM RESIDENT and cannot be changed or updated in the field.  Magellan (uniquely) can provide a service to change basemaps.  Basemaps in 2004 equipment do seem a bit more accurate than just a few years ago.

Example GPS receivers with BASE MAPS include but are not limited to:  Garmin G-12Map,  eMap, G-III PLUS,  StreetPilot,  Lowrance GM100 and AirMap,  and the Magellan 410.  The newer GPSRs such as the Garmin  G-76 series, iQue 3600, GPS-V,  LEGEND/VISTA,  SP-III, GPS-60C, GPS-76C/CS, StreetPilot 2600 series,   Magellan GOLD/PLATINUM  and Sportrak Pro, and RoadMate have somewhat better maps than earlier models BUT,  if you are looking for perfection,  you will be still be quite disappointed.

2)    World Maps:
            World maps are sometimes a step up in detail from base maps but are still reduced detail compared with normal street level maps since the entire world is covered on a single CDROM.  The user loads the CDROM on a personal computer and then uses the map displayed on the PC  to select areas for  upload to the GPS.  Generally,  the World Maps are purchased,  but again,  since low cost of the finished product is a significant issue,  compromises in map quality are inevitable.  The World Map quality is generally better than the base maps,  BUT,  you can still expect roads to be out of position in the range of 1000+ feet (300 meters) or more in some areas.  According to Garmin,  the WorldMap CDROM has not been a big seller and so they have not updated it as of January 2002.  All of the GPS receivers noted in item #1 above can accept their vendor's WorldMap except for Magellan which does not offer a WorldMap CDROM in  their product line.

3)    MapSource Maps:
        Garmin's MapSource City Navigator and City Select product lines (from NavTech Maps) offer (generally, but not 100%)  better quality maps than are available in earlier  MetroGuide, WorldMap or BaseMaps.  In the USA,  Garmin offers CityNavigator (SP-III) and CitySelect (GPS-V) as their top of the line maps which include automatic road routing capability.  The NEW MetroGuide USA (version 3) is next in quality and accuracy but,  (unlike the older MetroGuide version 2 and  ver 4.01 CN and CS)  CANNOT provide automatic road routing capability inside a GPS.  It can autoroute on a PC and you can download the route to your GPS.  "Roads and Recreation" MapSource maps are pretty obsolete and new buyers will want MetroGuide USA instead unless you have a GPS with only 1.4 megs of map memory.    "TOPOgraphical" MapSource maps offer maps with contour lines and land features at about 1:100000 scale.  Other Offerings in the the Marine area include FISHING HOT SPOTS and BlueCharts.  See information about these on Garmin's cartography website section.  In the rest of the world,  (excepting USA, Canada, Western Europe, Australia, and South Africa as of March 2004),  only MapSource Roads and Recreation maps are available.  Generally,  the MetroGuide and CN/CS maps are higher quality but still less than perfect due to Garmin's desire to keep the product sale price in the range of US$100.  In the USA,  these less detailed  maps are derived from US Government TIGER maps which are available royalty free.  Garmin (and many other US mapmakers) have updated and enhanced the original Tiger Maps,  and the offerings are generally accurate but with some notable errors here and there.

In the new MapSource Maps for Europe,  Garmin had to purchase maps from mapmakers in the various countries at whatever the best deal on offer.  Unfortunately,  map sources in OTHER THAN THE USA charge relatively high prices (and often,  ROYALTIES on each unit sold) for their maps and this cost must be passed on to customers.  Garmin purchased  maps  of the UK (and other countries) from various map vendors.  These R&R maps are of good quality,  but are not as detailed or accurate as the premium quality OSGB maps used in the UK MetroGuide Maps or the even more accurate NavTech maps used in the latest European CityNavigator and City Select Maps..  Since substantial royalties must be paid on these maps,  the cost for the UK maps is in the range of US$100 even though the UK is a much smaller land area than the US.  Rumor has it that a set of European CityNavigator maps are in the range of $600 due to the high royalty rates in that region.   MapSource maps will load into any of the Garmin GPS receivers noted in item #1 above among other Garmin units.  MapSource maps CANNOT be loaded into any other brand of GPS receiver.

Similarly for other European areas,  Garmin has purchased locally available maps for MapSource use and converted them for GPS upload.  The map areas covered vary somewhat but the accuracy is generally good (but NOT perfect) and you will see some spots were the GPS places you "off the road" by several hundreds of meters.

4)    MetroGuide Maps
        MetroGuide Maps were Garmin's premium map offering prior to the introduction of CityNavigator and CitySelect.  These maps will ONLY load into eMap, LEGEND, VISTA, G-76map and 76S,  StreetPilot III, StreetPilot (all), GPS-76C/CS, GPS-60C and ColorMap SP.  In the USA,  the high accuracy NavTech brand maps are used.  In the UK,   OSGB maps are used which do indeed provide high accuracy.  Even so,  you will undoubtedly find some areas where the GPS places you "off the road" by more than the nominal 15 meters specified by the GPS specification.  Some of these occurrences will be because the GPS has a measurement error in excess of the 15  meter specs.  This is because the specification is only a 95% confidence level specification.  This means that up to 5% of the time,  the error can be greater than 15 meters.  In addition,  there are undoubtedly MANY  areas on even these high accuracy maps where map errors will show a road out of its actual position by even a few hundred meters.

Similar comments apply to all of the MetroGuide Europe maps.  A new Mapsource MetroGuide map for Europe arrived in 2002.  This map offers the same maps as in CityNavigator and CitySelect (for automated guidance instruments like the G-V and SP-III),  but does NOT offer automatic "current position to desired destination address" guidance IN A GPS such as the G-V or SP-III.  However,  it is able to generate routes on the PC which can be downloaded into Garmin GPS receivers.  A new CitySelect/CityNavigator edition for Europe should appear in 2004.

5)      Canadian and Mexican Maps:
          Some jurisdictions charge MUCH more for their maps than others.  In Canada for instance,  Federal as well as Provincial and even Local governments closely control their maps and charge large royalties for their use.  Recently Mexico Maps has been offering Mexican uploadable maps to Garmin and other units both for Baja California and mainland cities.  For working examples, see

        Mapsource City Navigator Australia was released in June 2002 for use with the Garmin StreetPilot III.  This product autoroutes on both the GPSR (if the GPSR supports it) and the  PC. CNA has excellent metropolitan area mapping and includes most major rural highways. It does not have streets in many provincial towns apart from the main highway and lacks secondary rural roads.

        Mapsource Metroguide Australia was released in October 2002. This product does NOT autoroute on either the GPSR or the PC. MGA has the same metropolitan area mapping as CNA. It also includes streets in most provincial towns and most secondary roads. Cost is half that of CNA.  Both of these products are sourced from Sensis who have the electronic rights to the UBD maps.

Note; Both of these Australian products use a lock code which means they will not work with older GPSRs such as the GPSIII+ or older StreetPilot and SPCM  which do not have an internal ID code.

Joe Mehaffey                                             Back to Joe and Jack's GPS Information Website