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Need help getting custom maps onto Garmin Etrex 20 card...

Hello, folks,

I have been dragged (kicking and screaming) into the GPS age by demands of my work; up until now, I have been completely satisfied with my trusty Silva Ranger compass and 1:24K topo maps.

I have just purchased an Etrex 20 a week or so ago, and am learning my way around it. I have been able to successfully download USGS 7.5 min. maps to my home computer and convert them to kmz files using a couple different techniques, but the one that seems best to me so far is the G-Raster conversion program. Now, I would like to load my kmz map files onto a microSD card, so I can swap them out as needed. So far, though, I can only seem to load the map files directly to my Garmin, rather than the SD card which is installed, even though I have the machine set in “USB Mass Storage” mode.

I also do not understand everything I have read about the limitations of file size and “tiles”. I see here that some of you use 16G cards in your Garmins, but it appears that about three to five 1:24K topo maps exceed the 100-tile limit. Is there a way around this, or better yet, can you explain the “tiles” concept and size limitations in plain English or point me to a tutorial so that I can learn more on my own? Is this limit per card, per machine, or...?

What I would like to do is download multiple maps for use in searches for missing or injured subjects; my area of responsibility covers about 16 or so 7.5 min. quads. I would settle for being able to load up several cards with various sections, and just insert the cards I need at the time, but right now I can’t even seem to get the files onto the card correctly.

Any help would be most appreciated. Please be gentle- I have done a LOT of reading in the last few weeks, but am barely computer literate and even less so right now with GPS units.

Thanks very much!


  • Boyd 1953 Points
    The custom map feature is very limited. I have posted several of them here:

    Through trial and error, I found 10 feet per pixel is the best trade-off between resolution and tile size. At this resolution, the maximum area that can be covered with a custom map is about 20 miles x 20 miles (eg: 400 square miles). Due to these limits (imposed by Garmin's spec), your file should not be any bigger than about 35MB (that's MB and not GB), so you really don't need any kind of big memory card for this. The specs call for a maximum of 100 tiles, each of which can be no larger than 1024x1024 pixels. There is no way around this limit. And you have to physically swap cards to use more than one custom map.

    But really, making 24k topo's is a fool's errand now that Garmin has released Birdseye Topo. These are the same scanned USGS maps, available with unlimited downloads for $30 and you won't have any size limitations. Birdseye also is visible at a wider range of zoom levels and is faster at scrolling and zooming than custom maps.

    For Birdseye you may want a 16GB card. Personally, I have found it makes the GPS start up really slowly (on my Oregon and Montana) if you have that many files on a card, so I no longer use my 16GB card.

  • Hello, Boyd, and thanks very much for the reply.

    I did not know that the Birdseye Topo was unlimited in size. That certainly makes it worthwhile. I had not been able to find a lot of info on the Birdseye Topo program, except that it had a lot of problems when it first came out. So it is an exact copy of the USGS 1:24K maps, including structures, etc.? How do I get these to copy to the card and not the internal memory?

    Also if you don't mind, just for my knowledge, how do you control the pixel and tile size when making custom maps? I did one scan and transformation of a state park map overlay which worked fine on Google Earth, but my Garmin says it is to big to display. How would I correct this?

    Sorry to be so ignorant, but I do appreciate the help. Take care!
  • Boyd 1953 Points
    When you download the imagery, don't check to box to save to the GPS. When the download is finished, you can send it to the card. Before downloading, make a Garmin folder on the on the card and copy the GarminDevice.xml file from your GPS into this folder. You can now put this card into a USB reader and Birdseye will think it actually is your GPS. This allows you to download files while you use the GPS out in the field.

    I use Globalmapper for all my mapping projects. It's a very powerful but rather expensive program, so unless you want to spend around $350 on software my technique isn't going to help you much.

    Sorry, but I've never looked very closely at g-raster so I don't know if it lets you re-sample an image. There's another program called mapc2mapc that I believe will let you change the pixel resolution, plus some other cool thinks like making irregularly shaped maps to minimize the number of tiles.
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