Avenza Maps is a mobile app that shows your position on a geo-referenced map with your GNSS-enabled device (GPS, GLONASS etc). It can be useful when checking control placement, although it is not a substitute for accurate navigation and common sense!

Some history – many orienteering maps were made prior to the availability of Lidar base maps, so a little history lesson might give you an insight into the limitations of using GNSS devices with older maps that have been georeferenced after the original field work and map drawing (which could well have been a transcription of the original hand-drawn map into OCAD).

Prior to the availability of Lidar, base maps were made by photogrammetrists who interpreted aerial photographs to extract contours, plus whatever else was visible in the aerial photographs (roads, buildings, water features, anything else that the photogrammetrist thought could be useful to the orienteering field worker – e.g. prominent fallen trees on the ground). The accuracy/usefulness of this photogrammetry depended on:

• the accuracy and resolution of the aerial photographs
• the age of the aerial photographs
• the flying height of the aircraft taking the photos
• the skill of the photogrammetrist

Despite the inherent problems, the base maps used in Australia (produced mostly by Chris Wilmott specifically with orienteering maps in mind) were generally very accurate as regards contours, but features not available from the aerial photographs had to be placed by the mapper, another potential source of error.

So, what does this mean for the course setter when using Avenza Maps to check control placement?

• The position of a feature might be relatively accurate, but not absolutely accurate (e.g. correct in relation to features around it, but not in the exact location that a GNSS device declares it to be), so moving the feature on the map is not valid if this upsets relativities with nearby features.
• The temptation to add features should be resisted unless those features have been obviously missed, or are new (e.g. recently exposed tree-roots; new fences and tracks). Orienteering Tasmania’s policy is that updates to maps are carried out by the original mapper where possible. Just because the course planner thinks a feature should be added, it doesn’t mean that an experienced mapper will consider the feature to be mappable according to the mapping standards.
• Where the geo-referencing is inaccurate (for example, where multiple maps have been joined to make a composite map the join area might show some distortion), use common sense to select control features that are reasonably accurate from different attack points, or choose a different control site.

So, with these caveats in mind, how do you set up Avenza Maps?

1. The map must be geo-referenced.
2. You can check whether a map is georeferenced in various ways, depending on the software available to you:
   1. In OCAD, Map>Set Scale and Coordinate System will have the Real-world coordinates radio button checked.
   2. In OCAD Viewer, the top and side bars on the screen will have actual, rather than relative coordinates displayed (for example in Tasmania, these will be numbers in the 500 000 range on the top, and numbers in the 5 000 000 range on the side).
   3. In Condes, the relative map co-ordinates for the current cursor position are shown in the bottom right-hand corner of the screen, and for georeferenced maps the corresponding real-world coordinates will be shown in the box to he left of the relative coordinates (this box will be blank for maps that aren’t georeferenced).
   4. PurplePen does not appear to support the creation of georefenced images, however it does support the creation of GPX files from georeferenced maps, so it might be possible to generate a georeferenced TIF file in OCAD Viewer and add the control coordinates to the map from the GPX file, but I haven’t tested this option.
   5. Open Orienteering Mapper is another option that could be explored if your club doesn’t have access to OCAD or Condes.   
3. Condes
   After preparing your courses, export the controls as a georeferenced PDF file (Export>Export map and courses as PDF …, and select the All Controls check box). In order to minimise the file size, set the Print Area to cover the section of the map covered by the controls (and the control description as well as it is good practice to check the control description after placing each control). Although the file size will be quite modest when exported from Condes (it’s exported as a vector image), it is converted to raster format when imported into Avenza Maps and generally significantly larger.
   Although Condes supports the export of georeferenced PDF files, it’s possible to get slightly better resolution by exporting the controls as an OCAD file and generating a TIF file from OCAD, so if you have a highly-detailed map (detailed granite or mining), step 3.1 below could be an option, followed by step 4 (except for the course planning bit). won’t.
   1. Export the controls as an OCAD file (Export>Export courses to OCAD…). Open the All Controls map in OCAD (e.g. MyEventAllControls.ocd). The map will show only the control circles and control numbers, plus any other information added in Condes (e.g. control description, safety bearing), so the map must be added as a background map (Background map>Open…, e.g. MyCompetitionMap.ocd). Note that the map scale of MyCompetitionMap.ocd must be the same as MyEventAllControls.ocd
4. OCAD
   Set your courses in OCAD and Export a TIF file from OCAD with 600 dpi resolution (note that there is no need to select the Create World file (tfw) option as the georeferencing is embedded in the TIF file). To minimize the file size, choose the Part of map option to export only the relevant part of the map, e.g. MyCompetitionMap.tif will be exported.
5. Transfer the TIF (OCAD) or PDF (Condes) file to your GNSS-enabled device, or to a service accessible to the device (e.g. Dropbox).
6. In Avenza Maps add the new map (if you are using the free version of Avenza Maps, there is a limit of three maps that you can concurrently store on your device).
7. If all goes well, you will have a map in Avenza Maps ready to take with you into the bush!
8. Practice makes perfect, so practising on the nearest map to you before heading out into the bush makes a lot of sense.

Anyone wishing to try Avenza Maps, but without access to OCAD or Condes, can contact me for assistance.

Greg Hawthorne
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