Monday 13 February 2012

A little multiscale mapping gripe

Last week I had cause to travel from Las Vegas, NV to Mammoth Lakes, CA by car. There aren't many routes and so I neither bothered to take a road atlas with me or a sat-nav. Instead, I used a popular online mapping web site and plugged in my origin and destination to see which route was quickest and what difference to travel time the alternatives would take.

I hadn't traveled through Death Valley for a number of years so I decided to forego the slightly quicker route (further north) and head across on route 190. I checked the map at the following scale and knew I needed to look for a junction near Beatty, turning left onto the 190 from the 95.

Imagine, then, driving through Beatty and not seeing the junction. Odd. Given I'd clearly missed it I just traveled on, knowing I could take an alternative route but...what had happened to the 190? Later, i checked and found some annoying multi-scale map gaffes. At the next zoom scale you get this...

Now wait a minute...the single road going from Beatty, labelled 190 is now cut across by another road, some of which is labelled 190. So the 190 never reaches Beatty. That would explain why I never saw the what's the road? need the next zoom level for that...

You see, it's not the 190, it's the 374. In fact, you get three route labels just to make it clear.

So the lesson...I'm not sure there is one except sometimes you just can't trust maps, especially ones that make decisions on content generalization and labeling that cause fundamental problems for some map use scenarios. The first map scale just presents a distorted picture of reality. It's unhelpful and misleading and serves to remind us that maps need to communicate reality properly and that cartographic decisions (or automated procedures) don't always reflect reality.