By Richard Wilson
July 10, 2002
Revision 1, July 23, 2002

Only the user can determine the amount of memory required for the GPS receiver.  It depends upon the area to be covered, the type of usage, and the MapSource program.  Using Fishing Hot Spots to cover all the lakes in Kentucky requires only 1.7 Mb of memory.  To cover just the city of Los Angeles in MetroGuide requires about 30 Mb.  This article is intended to help make the decision as to the amount of memory needed for a particular application.

Most maps are in the shape of a rectangular block, and the dimensions of each block vary according to the amount of information and detail in that particular area.  The exceptions to this are Roads & Recreation where the maps cover a county, and Fishing Hot Spots where the maps cover a single lake.  The maps can be previewed prior to purchasing to let you see the area covered, and also the amount of detail in the map.  To see this go to and follow the directions for  MAP VIEWER  found on that page.  Unfortunately, this does not give the memory required for the individual maps.

Some of the Garmin GPS receivers have built-in map memory and the description of the unit on the Garmin site will give the amount of memory in the unit.  Others have no internal map memory, but use a data card that is inserted into a slot.  These cards are either pre-programmed, such as the BlueChart and Fishing Hot Spots maps, or blank cards that are user programmed from MapSource on CD-ROM (BlueChart and Fishing Hot Spots maps are available in both forms.).  The blank cards are available in 8, 16, 32, 64, or 128 Mb.

It takes about 12 minutes to fully load an 8 Mb data card using the serial port from a PC.  The computer must build a map set and index files and erase the card before it can load.  The whole process takes about 15 minutes.  The time increases proportionately with the amount of memory, so a 32 Mb card will take a almost an hour, and a 128 Mb card can take about 3 1/2 hours!  This assumes selecting enough maps to fully load the card, but loading just 8 Mb into a 128 Mb card will take about the same time as loading them into an 8Mb card.  The erasing may take a little longer in the larger card.

Data cards can also be programmed with a  USB Data Programmer,  available from Garmin or Garmin dealers.  The loading time is cut drastically and a 32 Mb card is fully loaded in a few minutes instead of an hour.  To load user defined waypoints and routes from MapSource to the GPS receiver still requires the use of the PC serial port.   These will load much more quickly than maps.  The data cards are only used to hold map data.

You can load maps of the same area from several programs.  For instance, on the trip you can turn on Waterways & Lights to see the details on that map, then switch to Roads & Recreation to see the information there.

The chart below gives some map size information that will help in determining the map memory size required.
Map Area 1

Largest Map

Typical Map

U S MetroGuide Rectangular Block 6.500 4 0.250 - 3.000
U S Roads & Recreation County 0.522 0.100 - 0.250
Fishing Hot Spots
Version 4.00
Single Lake or Section of Great Lake 2.290 5 0.020 - 0.650
Americas BlueChart Rectangular Block 6 See Below 6 See Below 6
U S Topo Maps Rectangular Block 0.598 0.200 - 0.400
Points of Interest Rectangular Block 0.403 0.010 - 0.200
U S Waterways & Lights Rectangular Block 0.475 0.050 - 0.300
North America 
City Navigator 2
Rectangular Block 3.110 1.000 - 2.000
North America
City Select 3
Rectangular Block 7.070 1.000 - 3.000


1. The map area varies depending upon the number of features and details in the area.  See the Map Viewer on the Garmin web site for the particular area of interest.  This is located at:
2. City Navigator is not for sale individually. It is included with Street Pilot III.
3. City Select is not for sale individually.  It is included with GPS V.
4. U S MetroGuide consists of over 1100 maps, the largest two being Alaska at 6.5 Mb and Hawaii at 3.7 Mb.  These maps cover the whole state, whereas the contiguous 48 states are broken up into smaller maps.  There are only a few of these that exceed 3.0 Mb.  If your GPS receiver does not support the auto-routing feature of MetroGuide (most do not), you can eliminate that feature in MapSource under preferences, transfer, and reduce the memory size required for the map by about 10 to 40 per cent.  If your unit supports auto-routing (for instance StreetPilot III, GPS V) you will get much better results if you load all the maps that cover your route, rather than just the start and finish.

5. Of the 300 FHS maps, there are only three over 1Mb:  Lake Eufaula in OK at 1.020, Toledo Bend Reservoir in TX-LA at 1.250, and Lake of the Woods in MN-ON at 2.290.
6. BlueCharts come with a number of maps per region and these maps are quite variable in size.  The list below will give you the total memory required for the entire region and the number of maps in that region.
Total Mb
1 7.300 35
2 8.900 59
3 8.300 81
4 9.000 94
5 9.500 83
6 10.200 78
7 9.100 90
8 7.700 62
9 8.300 91
10 10.700 110
11 9.200 61
12 9.900 80
13 8.300 74
14 8.400 97
15 3.700 63
16 5.500 152
17 4.500 108
18 6.700 191
19 3.300 47
20 N/A N/A
21 6.900 26
22 8.900 128
23 6.300 56
24 8.500 61
25 10.700 98
26 9.400 100
27 3.400 67
28 14.300 77

  The coverage of each of these regions can be viewed at:  Scroll down the page to the bottom.  The Americas BlueCharts are shown in groups.  These groups are only for clarification on the web site so that the user can easily see the desired region.  The unlock code will only allow you to see and use one region, numbered 1 to 28.  To get another region requires another unlock code.  This can be purchased through Garmin, or one of the dealers.

Some users like to have lots of maps loaded for anywhere they might travel.  They need to have quite a bit of memory for this.  I only load the ones I'm going to need on a particular trip and therefore don't need as much memory.  Decide what you want to do, add up the memory required for the maps you want to load, and this will give you the memory size required.

 Questions may be sent by e-mail to:  Richard Wilson

Click here to go to  Jack and Joe's GPS Information website