(meteorobs) OT- Map question. Clarification/How To

Mr EMan mstreman53 at yahoo.com
Thu Jan 1 16:08:35 EST 2009


Subject: Re: (meteorobs) OT- Map question.

If I understand the original intent, it is to use a large-scale map to plot ground tracks of fireball events.

I can see there is a lot of confusion going on here and with converting azimuths it is easy to get that way unless one has a broad understanding of all the concepts.  Map reading has many nuances and large scale maps aren't utilized for navigation and topo readings etc.

If you are hanging this map vertically up a wall, as I believe you said, all this discussion is not necessary as any orientation which is not level with the ground has nothing to do with plots and ploting using one's own observations.  That is unless you are overlooking the terrain you are working with comparing features then the map need not be oriented.  

 Nystrom's directions were correct if you were taking a magnetic north azimuth and converting it to true north in order  to orient the map level to the ground.  Central California's magnetic declination presently is 16° E meaning you nee to subtract 16° from magnetic north to align the map with true north. So rotate the map 16° counter clockwise after it is oriented to magnetic north in order for it to be oriented to true north. This holds for any map scale so long as it covers your physical location in Central California e.g Quadrangle Topo.

On large scale maps, magnetic declination diagrams aren't used because the declination varies widely from edge to edge and changes yearly.  Longitude lines ALWAYS point to true north and serves as the true north arrow. Only small-scale maps used in local orienteering events are printed oriented to magnetic north. Also avoid using Grid North/Lines for orientation to true north but that is yet another nuance.
 
If you are trying to plot ground tracks you need to know if the reported azimuths were given in true or magnetic: plot true azimuths directly and magnetic azimuths are ploted after being converted for their declination(not yours) value --both using a navigation plotter/protractor.  Unfortunately 3d maps are difficult to use with overlays and after several plots your map will be full of pin holes and erasures.

To find ground track data: 
1. If you have a start and end point of a fireball track you may plot the location of the observer  and draw an arc/fan but you will not be able to plot the ground tract without another set of points.  

2. You can compute a sanitized  ground track with USAF satellite observatons drawing a line between the two points. 

3. IF the observer was on line of sight then you can plot their reported azimuths as above but you cannot plot a ground track. 

4. You can plot the azimuth intersections of a single point, such as fireball fragmentation, with two to three observervations --IF you were so lucky to have two trained observers making accurate simultaneous measuements. Or if you can derive data from fixed point video recordings.
Hope this helps get things back on track.

Eman




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