LAND NAVIGATIONAL TECHNIQUE: AIMING OFF
The diagram accompanying this text is based on an overhead projector (OHP) acetate I produced over twenty years ago. On the acetate I used permanent OHP colour pens to help differentiate the various routes, signs and symbols. I can still smell the odour these pens gave off when I opened the cap! OHP’s were built to a military specification to survive a nuclear attack, remember the Cold War, but it had a weakness – bulb failure! Hopefully, if this did occur a spare bulb was located in it’s holder. Unfortunately, nine times out of ten it had already been used and one either had to locate a technician to get a replacement or swap the OHP with another one from the classroom next door. If the latter hoping that a lesson wasn’t in session. Times have changed technology wise in education during this period but the original visual information contained on that acetate still holds strong and the aiming off technique has not altered one iota. Back in the early 1990’s students would have received a black and white photocopy based on that acetate for their records and were encouraged to use colour pencils or fibre-tipped pens to highlight details whilst they copied from the OHP projection thrown onto a screen. Nowadays the information is stored digitally and the file can be printed as a hard copy in full colour, sent to a Virtual Learning Environment, e-mailed or viewed on a SMART board with all the associated interactivity this promotes. All wonderful stuff but the student still needs the ability to apply the aiming off technique in a practical activity. Only when this demonstration has taken place can the actual learning be evidenced. Now back to the topic area of the aiming off technique with a scenario based on the diagram and a rationale for using this navigational method.
Whilst on a walk in a moorland area you now have reached location L on your route (see diagram). The next part of your journey is to reach the end of the next leg which you decided will be a trig pillar. The weather has turned and low cloud is drifting over the area creating poor visibility and impairing your observational ability. Now you need to decide what navigation technique needs to be deployed to ensure you reach the trig pillar. As can be seen below there are a few possible outcomes to using a compass bearing.
D = Direct bearing from location L to trig pillar and hits target. Route distance is 1.5km.
A = A more likely route using a direct bearing from location L to trig pillar but error occurs. As you walk you drift to the left of trig pillar and miss the target – a possible situation.
Aiming off = The best option as the distance required for an accurate compass bearing using route 3 is, in this scenario, around a sixth of the distance using the direct bearing route from location L to the trig pillar objective [D].
Deploying the Aiming Off technique
Bearing [route 1] is taken from location L to forest boundary wall aiming away from the wall corner. Using this bearing the wall becomes a catching feature. Upon reaching it we know that we turn left and now use the wall as a handrail feature [route 2]. We continue to the wall corner and use this as an attack point. We can now take a bearing from the wall corner to the trig pillar and using this bearing walk to our objective [route 3]. This distance, attack point to trig pillar, is much less than the one from location L to trig pillar and therefore likely to be more accurate. To assist accuracy one needs to measure this distance and apply either the double pacing technique or timing to ascertain arrival.
Going back to route A how could you rectify your error? Assuming you are now at the arrowhead on route A you realise you have descended and by map reading note the forest boundary wall also turns down hill. Heading east [route B] you would reach the wall. At the wall you use this as a handrail feature turn right and follow to the wall corner [route C]. Once at the wall corner we now use this feature as an attack point and we can take a bearing to the trig pillar and follow route 3. You may also have realised your error because you did not reach the objective from either time estimation or pace count calculation. Here you need to stop when the error has been realised and either start a sweep search or deploy the aiming off technique using routes B-C-3 stated above.
Rationale for using the Aiming Off technique
When using a base-plate compass care must be taken to ensure the bearing is accurate. However, they are calibrated to 2 degrees and it is easy to move the compass housing to one calibration either side of the intended reading (1). This gives an error of 2 degrees and can easily be more if one is trying to follow that bearing in adverse weather e.g. high wind, rain, mist and snow. Another 3 or 4 degrees error can be added in these conditions which can cumulate to a 6 degree error. The effect of a 6 degree error over a distance of one kilometre is that you will be miss your objective by 105 metres (1)! This has a serious consequence if visibility is only 20 metres. Using this calculation the percentage on a 6 degree error is just over 10%. Worth keeping in mind for the next time you need to use a compass bearing. Ways to avoid compass bearing errors are to be accurate as possible and to keep the distance of the bearing as short as possible. As this cannot always be the case this is where the aiming off technique comes into play. In the example on the diagram the distance from the attack point to the objective [route 3] is less than a quarter of that from location L to the trig pillar objective [route D] thereby limiting potential errors.
Ordnance Survey Triangulation Stations
As the navigational objective of this scenario is a triangulation pillar it is worth mentioning briefly what they are. Triangulation stations were set up in a variety of locations in Britain by the Ordnance Survey for the purposes of mapping. The preferred location was on high ground with a 360 degree view. There are a number of categories of triangulation stations and the most visual is a type of ground station known as a triangulation pillar or trig pillar for short. Triangulation pillars are made from concrete or can be a natural stone pillar (2). They are 1.2m high with a brass plate set into the top to accept a survey instrument (2). The coordinates for triangulation stations have not been realised via ETRS89 and OSTN02. They are the original archive coordinates and can therefore no longer be considered as true OSGB36 National Grid Coordinates of the station (2). The Ordnance Survey no longer uses triangulation pillars as this method of surveying is obsolete. Those that are of a historical significance or located in a site considered to be important are still cared for but many others have been left to decay naturally. However, trig pillars can still be found in their original setting generally in a weathered state and are still used as location indicators. It was a trig pillar’s role as a location indicator that provided the idea to use one as a target for the aiming off technique over twenty years ago. Trig pillars are still in situ today but don’t expect them to be in a gleaming new coat of white paint.
Link to the image: Aiming Off
1 Peter Cliff Mountain Navigation (1978: 21/22) latest version (2006) ISBN 9781871890556