© Geoff Radcliffe 2011

 

Walking with a GPS Device

Map Grids and Datums

When using a paper map with your GPS device it is important to consider its coordinate system (the grid system and datum used). This is used to determine the position of any location shown on the map. Often the grid and datum is printed on the map. Any mapping software you are using should be set to use the corresponding datum and grid. For instance in the UK, maps normally use the British National grid and the Ordnance Survey Great Britain (OSGB) datum.

In other countries, a lot of the recently produced maps that have GPS support use the UTM grid and WGS 84 datum.

Once you know the datum and the grid used, you should ensure that your digital map software is using the same grid and datum as the paper map you are working with.

If you don’t know the grid and datum of the map, then you can find the commonly used ones by country at eye4software.com.

User Defined Grids

If your map software doesn’t have a suitable grid already defined but it allows you to define your own grid, then providing you know the parameters required you can define waypoints that use the same grid system as your paper map.

Garmin Mapsource needs the following parameters to define a grid:

The values for these parameters may be found using sites such as eye4software by searching for the appropriate country.

Determining values for your own Grid

If you cannot find suitable values for the grid parameters, the following method should give reasonable results. This method works with Garmin MapSource but it should also be able to be used with other map software that allows a user defined grid to be used.

Download the sample spreadsheet calculator from here.

Start up Google Earth and add a folder to the Places panel and give it a suitable name e.g. MyMap.

Find a location on the map as near to the top right-hand corner as possible which would be easily identifiable in Google Earth such as a landmark, part of a coastline or river, or a road junction and make a note of it’s grid position. In Google Earth find the location and create a placemark in the new folder (MyMap) as accurately as you can.

Find a second location on the map as near to the bottom left-hand corner as possible which would be easily identifiable in Google Earth and make a note of it’s grid position. In Google Earth find the location and create a placemark in the new folder (MyMap) as accurately as you can. Save the contents of the folder as an .kml file and use GPSBabel to convert the .kml file to a suitable format for importing into your mapping software (e.g. A Garmin database for Mapsource).

In Google Earth find the longitude of the leftmost placemark you added. Round this figure down to a whole degree. This gives you a starting value for a Longitude Origin.

In the spreadsheet:

In your map software, view the imported waypoints and set the map datum and the grid to be User Defined. To do this in MapSource select Preferences... From the Edit Menu and choose the Position tab. Ensure the Datum is set and select the User Defined Grid and press the Properties button.

In the setup for the User Defined Grid enter the value for the Longitude Origin, set the scale factor to be 1 and set the False Easting and False Northing to zero.

MapSource should now give grid positions for the locations. If not, try entering a False Easting of 10000. If the locations are within one degree (latitude) of the equator, enter a False Northing of 100000. If the locations are south of the equator, calculate roughly how far south (using Google Earth) and round up to the nearest 100 km. Enter this value as metres into the False Northing.

Once, you can view the coordinates of both locations in MapSource, enter the False Easting and False Northing values you have used into the spreadsheet.

Enter the coordinates of the two locations into the spreadsheet under the corresponding longitude column.

Repeat the process for the other longitude values, setting the Longitude Origin and entering the new grid positions into the spreadsheet.

From the spreadsheet, choose the column that gives the least X-Y variation and enter its longitude value as the Longitude Origin to use and enter the Calc Scale Factor into the Adjusted Scale Factor.

The spreadsheet should now display the False Easting and False Northing to use together with an estimate of the accuracy of the grid for each of the longitudes.

Using the values for the column that gives the least X-Y variation, in MapSource set the User Defined Grid to have the values of the chosen Longitude Origin, the Adjusted Scale Factor, the False Easting and False Northing.

MapSource should now give the same grid values as the paper map. You should also be able to use these values in your GPS device to give you your position using the same grid as the paper map.

Links:

Convert Coordinates - Calculate a position in a variety of formats

http://www.earthpoint.us/Convert.aspx

 

For documentation on settings for user defined grids and datums by country see:

http://www.eye4software.com/resources/grids/