Sunday, March 18, 2012

What kind of maps can I put on my GPS (and what do you mean by raster and vector)?

I received a PM from the Adirondack Forum today that said:

"…I just bought the Garmin 60CS. I have a question about the maps. Do I need the Garmin Mapsource United States Topo or… the one from National Geographics Topo that I think is better for hiking? If I don’t buy the Garmin Mapsource Topo, can I download maps in my GPS?…"Raster_image

This question comes up so often that I felt obligated to post an answer here. The answer is no, you cannot load National Geographic TOPO! maps onto any GPS. You can however, create waypoints and routes with TOPO!, and transfer these to your GPS, but not the maps. Let’s look at why not, and what kind of maps you can load onto your GPS.

The National Geographic TOPO! maps look just like a paper map from USGS (example above, at right). That’s because they are scanned images, also known as raster imagery. Since they are scans, they have a pretty big file size. That’s why it takes 10 CDs to cover the state of California. And that’s one of the main reasons that GPS manufacturers have taken a different approach.

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Does heavy cloud cover affect GPS reception?

Here, in our first best of the boards post, I’m quoting from alt.satellite.gps. The question that was asked  was, does heavy cloud cover affect GPS reception?

Dave Patton, Canadian Coordinator of the Degree Confluence Project had posted the best answer as of this morning…

The GPS signals will not be affected by clouds, rain, fog,
snow, etc., because of the combination of their passing
through water vapour, and the signal wavelength meaning
they "pass through" rain etc.

On the other hand, water will block/attenuate GPS signals.
For example, you can’t receive GPS signals underwater, although
having a GPS receiver antenna very close to the water surface
may allow some reception.

Most cloth that would be covering a GPS receiver’s antenna
will pass GPS signals(e.g. backpack, jacket, etc.), but cloth
with a metallic component will block/attenuate the signals.

Cloth that covers a GPS receiver’s antenna and gets wet
(e.g. from rain) can also block/attenuate the GPS signals.
In fact, you don’t even need the cloth – water on the case
that covers the GPS receiver antenna can block/attenuate
the GPS signals. That water could be drops, and/or a film
of water. I’ve seen that happen myself, when using my etrex
Venture in the rain – the signals were weak/blocked, but
a quick drying-off of the GPS case over the antenna brought
the reception back, which then degraded again as water
accumulated on the case. Similarly, GPS signal reception
is degraded in wet forest canopy conditions compared to
when the forest canopy is dry.