Ever since Garmin released Custom Map support on the Oregon, Dakota and Colorado this morning I’ve been playing around with this nifty new feature on my Garmin Oregon 550t. Below I’ll try to pass on a few tips and show some examples to get you started.

First I started with a trail map for one of our local (but well known) State Parks. The trail map was in the form of a .gif file so I converted it to a JPEG image and began to follow the instructions on the Garmin site.  Garmin’s instructions are very easy to follow and within about 10 minutes I had my first map loaded onto the Garmin 550t — not bad.

Here’s the JPEG I started with.Walden Pond map

Here are some of the screen shots of the custom map on my Oregon.


Although I haven’t tested the accuracy of this particular map, a second map I made for a conservation land near home was very good, my tracks were within 10′-20′ of the trails on the map.

On the GPS unit there really is not any difference between using custom maps and Garmin maps.  You have the ability to enable and disable custom maps just like any other map layer under Setup>Map>Map Information.  All of your maps show up under a button named “Custom Maps” and the name of all the KMZ files which are loaded are visible inside the button.   Custom maps loaded in either internal memory or the SD card (I’ve tested both) will appear on the map page and are enabled and disabled using the same button.  Note: you do not have the ability to enable each custom map separately, they are either all enabled or all disabled. This is an obvious feature improvement that I’m hoping Garmin will consider.

Features (eg. lakes, roads, contours) on the underlying Garmin topo map are selectable through the custom map.  If you notice in the first screen shot image above I’ve selected Walden Pond which is a POI on Topo 2008 and is not part of my custom map.

Garmin Oregon map setup pageMap enable for custom maps

There are several aspects of the Garmin Custom Mapping procedure that are important to understand because they affect quality of the map you see on the GPS. The first is obviously the accuracy of the map calibration in Google Earth.  Here are a few tips to help you get the best calibration possible:Google Earth overlay controls

  1. Start by zooming into the area on the Google Earth map where you are going to locate the map image. Your viewable area should be roughly the area covered by your JPEG, maybe slightly larger.
  2. Center the map first by grabbing the recenter tool in the middle of the image (green “X”)
  3. Rotate the JPEG (if required) to get the rotation correct using the rotation tool (look for a handle on one of the green side “T’s”)
  4. Use a shift-click to grab a corner of the image to stretch it into place. Using a shift-click vs. a click will maintain the aspect ratio of the photo
  5. Fine tune by rotating or resizing

The second thing which matters is the resolution of the map relative to the real land area it covers.  This will determine the zoom levels where your map is readable without excessive pixelation.  The map I calibrated above was good down to about the 300ft zoom level.  To determine the quality of your maps you need two things:

  1. The pixel width of your JPEG image.  This is the first number in the resolution of the image, usually available under image properties.
  2. The actual land distance spanned by the top edge of the map in feet (or meters).   You can use the measurement tool (Tools>Ruler) in Google Earth to come up with this number once you have completed the calibration of the map.

Now divide the land distance spanned by the map from 2) by the pixel width of the image from 1) and compare that to the chart below.  Using your ft/pixel (or m/pixel) value search down the “map ft/pixel” (or “map m/pixel”) column until you find the first value that is larger.   That is the most zoomed-in level where you won’t see any pixelation.  In practice I find that one more level zoomed-in is visually acceptable but any more than that and you’ll start to notice jagged lines.

GPS zoom level map ft/pixel GPS zoom level map m/pixel
20 ft 0.4 5 m 0.1
30 ft 0.6 8 m 0.2
50 ft 1.0 12 m 0.3
80 ft 1.7 20 m 0.4
120 ft 2.5 30 m 0.6
200 ft 4.2 50 m 1.0
300 ft 6.3 80 m 1.7
500 ft 10.4 120 m 2.5
800 ft 16.7 200 m 4.2
.2 m 22.0 300 m 6.3
.3 m 33.0 500 m 10.4

In addition Garmin also recommends that you keep each JPEG image to less than 1 megapixels (e.g. 1024 x 1024 or 2048 x 512), otherwise the unit will render the image at a reduced resolution.  Their suggested solution is to break the map in multiple <1 megapixel chunks, noting that multiple images can be embedded in the same KMZ file, but that’s sounds like another post on more advanced techniques.

As an example, if you want a square map that looks good at the 200′ zoom level you should be trying to get a 1024×1024 image that covers a little less than a square mile (1024 x 4.2 = 4300′ per side).

Hopefully that will keep everyone busy until I can put a post together on how to get USGS topos and aerial images onto the unit, I’ve done this already using some existing tools.   More on that in the next post.