I live in northern British Columbia where the relevant declination values are currently
- Magnetic declination: 17° 45.30' East
- Grid declination (for Natural Resource Canada's NTS maps): 16° 17.04' East
I am fairly comfortable using my compass to both 1) acquire a bearing on the ground to plot on a paper map and 2) read a bearing from a paper map to follow on the ground. I understand the importance of having the (roughly) correct declination when transferring a bearing between a map and the real world in either direction.
I believe that grid declination is specific to the mapping product, and a non-NTS map for the same location could use a different Grid North. As a result using my compass with a different map, without changing its declination, could result in navigational error.
My question is for help explaining the following scenario: a colleague had an iPad showing a polygon overlaid on an app using the Web Mercator CRS (900913 / 3857 / 3785). We were starting at one corner of the polygon and our goal was to track along the edge of it. My colleague placed the compass on the iPad screen (held parallel to the ground), aligned the compass's grid lines with the orientation of the map (grid north on the map was parallel to the screen's side bezels) and read the bearing. We followed the bearing and - to my surprise - closely tracked to the desired edge of the polygon. I would expect some risk of magnetic interference from the iPad, with the compass directly on the screen, but this did not appear to be an issue.
Given my belief that grid declination is specific to the mapping product's Grid North I do not understand how this approach was successful.
- was it purely a coincidence?
- does the Web Mercator CRS happen to use the same Grid North in my area (NTS maps use NAD83 UTM)?
- is it possible that error introduced by reading the bearing from the iPad and navigational errors in following the bearing were simply too small to matter, or potentially cancelled each other out?