Bearings using drawing tool

[Larry]

Hi Scott, hope you are having a nice trip!

I normally use NGS Topo! to generate the range and bearings I use in making my labeled panoramics, but Topo! does not cover Canada, so for my latest pan from the North Cascades, I'm trying to use TF to ID peaks across the border. For some reason, the bearings reported by TF did not add up with the pan (as do those from Topo!) so I did a quick check:

From Kitling to Joker, Topo! reports 357.7 deg (TN), while TF reports 356.1 deg. How does TF arrive at this figure? Could it be grid north vs true north? Is is a constant offset at least for a small area?

Any help greatly appreciated. I think I saw something about this in another post, but is it possible to get the elevations of contours on the Canada topos? It's rather hard to figure out what's going on without them.  '>

Larry

[ScottMorris]

Larry--

Sorry I missed this post.  

The bearing reported by TF is indeed grid north.  I'm not sure if that accounts for the discrepancy or not.   What do you mean by the bearings not adding up?  Regardless of the definition of north, it's still 360 degrees and should sum correctly.

Maybe I'm missing something.

Regarding elevation contours for Canada -- unfortunately there is no way to turn them on in the server.  I'm hoping to get into elevation caching with TF soon, though, so there's the possibility we could generate them.  '>

[Larry]

Scott, if bearings reported by TF are grid north, that explanis the discrepancy. By 'not adding up' I meant that once I get my scale overlay calibrated, peaks on my 360 degree pans match up with the bearings reported by NGS Topo! to within 0.2 degrees now. (This took some doing by the way. :-) Using the bearings reported by TF then did not correspond. Topo! bearings are all true north.

In this general area, the difference between grid north and true north is 1 plus degrees, depending on the USGS topo location I gather. I think this explains the empiric 1.6 deg I found.

In that GPS units report bearings in reference to true north, and that grid north appears to be map specific, is it possible for TF to report bearings in relation to true north (TN)?

I wonder if any others have spotted any discrepancy. Say with  a gps generated waypoint to waypoint bearing that does not agree with that calculated by TF.

I'm not sure of other fields, but for marine navigation, bearings used are either TN or MN, never grid north. At least we never used GN.

Larry '>

[ScottMorris]

Larry,

Thanks for politely pointing out that TF is basically computing the wrong thing.

I changed it to compute true north (basically doing the angle computation using lat/lon instead of UTM coordinates).  It'll be in the next beta.

Thanks.