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 of Orienteering Tasmania’s 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 geo-referenced some time 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 were generally very accurate as regards contours, but features not available from the photogrammetric base map 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). OT’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 mappable.
- 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?
- The map must be geo-referenced (you can check this in either OCAD or OCAD Viewer if you don’t have access to OCAD). In OCAD, Map>Set Scale and Coordinate System will have the Real-world coordinates radio button checked. In OCAD Viewer, the top and side bars on the screen will have actual, rather than relative coordinates displayed (numbers in the 500 000 range on the top, and numbers in the 5 000 000 range on the side for Tasmania).
- After preparing your courses in Condes, export the controls as an OCAD map (Export>Export courses to OCAD …, and select the All Controls check box). If you are familiar with OCAD course setting, setting the courses in OCAD allows you to bypass the Condes-OCAD step.
- Open the All Controls map in OCAD (e.g. MyEventAllControls.ocd)
- The map will show only the control circles and control numbers plus information added in Condes (e.g. control description, emergency contact info), so you must add the map.
- Add the map as a background map (Background Map>Open… – e.g. MyCompetitionMap.ocd)
- MyCompetitionMap.ocd should align perfectly with the controls as placed in Condes. Note that the map scale of MyCompetitionMap.ocd must be the same as MyEventAllControls.ocd
- Export a .tif file from OCAD, with 600 dpi resolution. 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.
- Transfer the .tif file to your GNSS-enabled device, or to a service accessible to the device (e.g. Dropbox).
- In Avenza Maps add the new map (if you are using the free version of Avenza Maps, there will be a limit of three maps you can concurrently store on your device).
- If all goes well, you will have a map in Avenza Maps ready to take with you into the bush!
- 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, can contact me for assistance.