Street Maps
 

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To get to the trailhead, or go off on a Geocache hunt, one obvious option is to use your GPS receiver along with some street mapping software. Metroguide maps, which only work with Garmin receivers are often said to be the best for use in the rural sort of areas that you'll be heading to. In fact, the group think on the GPS newsgroups often makes it sound as if these maps are extremely up to date and accurate, and you'll often see people getting flamed if they try and say otherwise. Unfortunately, I've found that Metroguide maps are in fact quite variable in regards to the age of the street data, the road layout accuracy, and road positional accuracy. Knowing in advance whether the maps are any good or not can be quite hard to do. At least a third of the Metroguide database is quite obviously simple census bureau tiger map data, which isn't always as reliable or accurate as you might desire. In the 30% area, Metroguide is also derived from an older version of the Tiger map data than what you'll find in Magellans Tiger Map sourced Street maps. If you go to the Tele Atlas website, which is the company that provides the data for the Metroguide maps, you'll find that they claim accuracy of 12 meters or less for 70 percent of the streets where most people live. I'm not sure what exactly that is supposed to mean, but it appears to mean that the major cities are up to date, the rest of the country is tiger map data. From what I've been able to determine, there are updates to freeway and other major roads that appear to reflect road layout as recent as 5 to 7 years ago. For most of the residential and rural areas in the Utah, Arizona, Idaho, Wyoming, Nevada and Colorado area though, the data seems to reflect the way things were somewhere around 1985 to 1990. The Magellan Mapsource data appears to reflect road layouts from around 1990 to 1995. Otherwise it's the same as what you'll find in R&R, or the 30 percent of Metroguide that is Tiger Map data. As a general rule of thumb, if you're in the 70% area of Metroguide, it's more accurate, detailed, and up to date than the Mapsend Maps. If you're in the 30 percent group for Metroguide, the Mapsend maps are better, but of course you have to have a Magellan receiver to use them, and the positional discrepancies will be the same as Metroguide, with the difference being the addition of some somewhat newer roads. Bottom line you're stuck with what will work in your receiver, but it doesn't really matter as the other guys maps are probably just as bad about the same amount of time.

As to what to expect for positional accuracy, it varies quite a bit. Around 300 feet seems pretty common in the residential areas around here, but a quarter of a mile isn't all that uncommon. Then there are the really big errors, where it's hard to say what's going on as you can view by following this link. Of course quite often the maps are dead on as well, particularly in the large cities, so if you're doing the city tourist, or business sort of thing, the maps can be pretty good. I'm not including any sections regarding the areas where the maps are accurate. Those areas are self explanatory in that the roads are as shown on the map. For hikers, and others heading to rural or remote areas, the maps can be painfully inaccurate.

Here is an example of city data I collected with a GIII+ from Ogden Utah. At the time, I was driving around looking for the best place to park in order to look for a geocache (I could have read the printed directions, but that takes half the fun out of it) It's hard to say how representative this particular example is due to the wide variations in the accuracy of the maps. I can easily find areas that are more accurate, and I can also easily find areas that are worse. This map pretty much reflects what has come to be my expectation of metroguide however.

Garmin Metroguide Example

Figuring out what street you're actually on isn't always easy due to both the inaccuracy, and the age of the maps. Turning the lock to road feature on in an area like this will often times really confuse your receiver, and using pre-planned routes can result in more confusion than help. In this case I found the Metroguide maps to not be particularly helpful for getting me close to the cache.

Magellan TOPO example

The above map is from Mapsend TOPO. As you can see, it's more modern and shows roads that Metroguide doesn't. Unfortunately, the inaccuracy in regards to position makes navigation on the GPS with these maps difficult.

Here is an aerial photo map of the same area:

Photo map using expertgps

As you can see, the receiver appears to have been fairly accurate in recording the track. There are however roads on the picture that don't exist in Metroguide, and the street layout and positional accuracy of both Metroguide and Mapsource could use a little work. Judging by the trees, houses, and other things along some of the roads I was driving on, much of this development is fairly new, probably about 10 to 15 years old. I didn't do any research to try and get an exact date of when it was added though.

I've found that Freeway data isn't always as accurate as you would like either. As an example, below is a tracklog displayed on Metroguide from the I-15 to I-84 interchange.

Garmin Metroguide Example

Believe it or not, this is a significant improvement from what R&R showed for this section of freeway. Having the intersection represented in the wrong place by more than .2 miles however can cause problems if you're relying on the compass page with a pre planned route for navigation. Often times the receiver won't tell you about the turn until you've already passed it. I've found this to be somewhat of a problem on major roads, and a significant problem for smaller city streets.

Garmin R&R example

Above is what R&R shows. Where they got the data from, who knows. Wherever the data came from, it's so inaccurate as to make navigation tough:

Magellan TOPO example

Looks likes Magellans somewhat more modern Tiger Map data is the most accurate in this case, as you can see from the map above.

Of course as the baseline to ensure GPS tracklog accuracy we have the photo map, shown below.

Photo map from ExpertGPS

Once again it would appear the GPSR was quite accurate, and that the problem lies in the maps.

On the Garmin maps, Metroguide does have better search functions than TOPO, or R&R. As mapping software goes though, the searches are quite crude. If you don't type what you're looking for into the search field in just the right way, you're not going to find what you're looking for. As a result, I've found it can be very frustrating trying to find things using the Mapsource software. The latest version of Metroguide USA also has an auto routing feature which can be very useful for building routes on the computer to load into your GPSR. Mapsource TOPO from Magellan has road data that is somewhere in between Mapsource and R&R in regards to age and accuracy. Which is better depends a lot on the area you intend to navigate.

All in all, I find electronic GPS maps to be a mixed blessing. It's unfortunate that there isn't anything else by other manufactures can be uploaded to my GPS units. For use on the computer, the software is crude, with Mapsend being harder to use in my opinion than Mapsource. When using electronic maps in the field with your GPS receiver, it's hard to know in advance what to expect. At times, the maps will lead you flawlessly right to the doorstep. Other times it's going to cause you anguish, and frustration due to outdated, and mis-located street data, which for me has led to my mistrusting the GPS electronic map combination. The more I've used the GPS on the roads, the more I tend to think the old paper map / road sign combination isn't so bad after all.

For what it's worth

Jeff