参考 http://garminbasecamp.wikispaces.com/Custom+Maps
自制地图是KMZ文件，其中打包了jpg图片作为地图显示。限制：
- More than one JPEG can preside in a KMZ file.
- The only KML file that will be processed is the doc.kml, inside the KMZ file.
- Jpegs files are the only supported image type (Single Layer Only - No Progressive JPEGs).
- KMZ files are read from the [Device]\Garmin\CustomMaps\ directory from both the GPSr internal memory and the µSD card.
- Images over 1 mega pixel (1024x1024 pixels, 512x2048 pixels, etc.) will be rendered at a reduced resolution on the unit. If this is causing a problem for your map, you can split the image and use multiple JPEGs inside of one KMZ file.
- Each JPEG should be less than 3MB.
- Most Garmin GPSr will index a maximum of 100 Custom Map JPEGs.
- The size of the JPEG files will affect the draw speed on the GPSr.
- There is no limitation on KMZ file size so long as the previous limitations are observed.
每个KMZ中最多包含100个jpg图片，每个图片文件最大3MB，每个图片最多包含1M像素
Garmin eTrex 20/30 自制地图（KMZ）的限制
原文：http://www.cnblogs.com/pffar/p/garmin_etrx20_30_custom_maps_restriction.html
