NZRM GPS Projects

One of the greatest technological advances of the last 50 years in mapping has been the advent of GPS, the Global Positioning System originated by the US government in 1973. The first GPS satellite was launched in 1978 and since then, the GPS system has offered major benefits for navigation especially with the present day handheld devices we can use for this. There are of course now a number of similar systems in use with various different governments creating their own equivalents, such as the GLONASS system used in Russia, and the others that are being devised by the Chinese and the European Union. GPS itself relies on a constellation of satellites which are located roughly 20,000 km above the earth, which makes the technology quite amazing when you consider the signals can be received at such a huge distance.

The NZ Rail Maps Project sees the benefit of using GPS data to help users find their location relative to the project’s maps, i.e. in proximity to points of interest on these maps. This is of greatest use particularly for ghost (closed) rail lines when it is actually difficult to find features of these lines otherwise, due to the changes in the topography of the ground where the lines have previously operated, due to the passage of time. The Project expects to make this possible with the smallest, cheapest GPS units on the market, due to the issue that most of our users are not likely to be in the business of using GPS a lot and will not wish to spend more money on a higher end unit. This means essentially that available GPS data from the Project will only make use of the Locations layer, which pinpoints stations and kilometre posts, rather than actual routes taken by lines.

One of the cheapest GPS units on the market is the Garmin etrex 10, which typically retails for slightly less than $200 in NZ. This makes it a desirable unit for the purposes of low cost mapping for the NZ Rail Maps Project. According to the capabilities of the eTrex 10, the best way for the Project to work within these capabilities is to export the Locations layer in each of the 12 volumes as a separate file, one file per volume, by which each location can be produced as a single waypoint with an appropriate name which is composed from the local name (if any), distance (if any) and line code. It is understood that a maximum of 1000 waypoints can be loaded into the eTrex 10 from a provided GPX file that can be loaded onto the device via its USB interface. So far there is no issue with generating these GPX files directly from a QGIS map file; the 1000 point limit having not been exceeded in any file created to date (there is only a possibility of approaching this with Volume 2 and this has not been confirmed at the time of writing this).

Field trips having been undertaken in both Volume 10 and Volume 11 areas, the results were found to be the same each time. The current location being viewed on the GPS’s screen was found to be able to match up to the waypoints loaded from the relevant GPX file very accurately. In both cases a new layer was created directly in the Qgis project and given a single data field called “name” with a string data type of 80 characters maximum length (this is typically shorter when loaded into the GPS) which is made from four fields that are joined together (the source layer’s name, caption, distance and unit fields). The layer was then saved into the GPS file format after being given a CRS of EPSG:4326 which is the correct CRS for GPX. The resulting file can then be copied onto the Garmin eTrex 10 GPS using its USB interface. When the eTrex 10 is connected to a Windows PC, it appears as a mass storage device with a Garmin folder within it. In this folder, there is a GPX folder, into which the GPX file created from Qgis is copied. As soon as the GPS unit is disconnected and turned on, the waypoints will appear on the map screen. In addition to allowing navigation via waypoints, the eTrex 10 also records the actual track taken by the user in travelling, which can be downloaded later in the Current.gpx file in the Current subfolder of the GPX folder on the GPS unit. This enables the user to determine their exact location during their trip, which is useful for correlating where they may have undertaken a specific action, such as taking a photograph.

The GPX files for Volumes 10 and 11 will soon be uploaded onto the Volumes site making them available for download by anyone.



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