- Digital Charts of Lough Corrib


Go to content

Getting started - Google Maps and Shorelines

How the Chart is made

Click on images for full-size preview (Click image again to return to website)

The simplest way to produce a quick chart is to use Google Maps as the basis. There are however some serious problems to be overcome before a reasonably accurate chart can be created using this data

How to correct Google maps position errors
Google maps does not contain accurate enough data to enable it to be used directly for a homebrew map. This is because of projection and rectification factors. You can see the amount of error in the image below, in the area we are dealing with, a position from Google Maps is in the order of 10 metres in a direction approximating East from its actual position in WGS84 datum

A good gps tracklog of a prominent object is needed. In this example, a jetty is used which we were able to walk slowly around with the gps set to log at 1 second intervals. EGNOS was enabled and active, and the HDOP was checked which was reasonable (it was actually less than 1.0). The tracklog is then saved to PC using Mapsource

Next the gpx file is opened using GPSMapedit

GPSMapedit needs to be configured so that "snap to grid" is disabled. This is very important as data will move around each time if the program is launched if this setting is left to 'on'. This is OK for charts which are only going to be used as rough guides, but if the program is being used to produce data for further editing, leaving this setting 'on' makes a mess of things

The next step is to use the “Show Googls Maps” feature to open up a photographic background and draw a polyline to represent the shoreline you are mapping (ignoring your gpx track). Here you can add rocks, buildings, obstructions etc

Now choose a part of your tracklog that you know is accurate and is easy to correlate with the photo background (In this example the SE corner of the main pier is used as we know that it is the section we concentrated on getting a good baseline along the very edge of the pier). Disabling “Show Google Maps” makes things easier to see, and select all the objects which have just been created except for the tracklog

Next zoom in so that the tracklog and the shoreline which you want to overlay can be seen

Now, if all new objects are selected, they can simply be dragged until the shoreline correlates with the tracklog

And there you have it, a georeferenced shoreline!

Back to content | Back to main menu