Re: Question about the maps on OWC and OW
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Hi Ted,
This is a great question. The selected point on the white chord line represents the time displacement between the occultation time of your selected location at mean see level and the actual elevation corrected time of occultation if the white chord was at a mean see level.
This could be seen as extra information noone cares about as observers plan for their expected cords relative toi each other, not their expected time of occulation, but it felt that if I drew the shortest perpendicular line, this would be incorrect and misleading.
Hristo.
On Monday, August 19, 2024 at 11:07:24 PM GMT+10, Ted Blank <tedblank@...> wrote:
Hi Hristo, thanks for the explanation.? I have one more question, how did you choose the location to place the end of the white line?? Why is it at an angle to the altitude-corrected chord location, and not drawn along the shortest path perpendicular to the (adjusted) chord, like the red line in my sketch below?
Thanks,
Ted B.
On Sat, Apr 20, 2024 at 6:49?PM hristo_dpavlov via <hristo_dpavlov= [email protected]> wrote:
Hi Vince,
What you see is the way OW and OWC show you the effect of altitude correction. You actual cord position is close to the shadow edge. The small difference between OWC and OW plots are likely a result of either rounding or perhapse if you have manually entered your altitude in OW dekstop and it differs from the automatic altitude that OWC is getting this could also explain the small difference. Regardless your actual chord position is close to the edge of the shadow.
> An finally,? what do I, as an observer need to do to make everything line up correctly?
Nothing. This is not meant to line up correctly. The real qustion is how you can interpret the white line and how you could select an observing location at a chord that you want.
Let me start with the white line in OWC. What you see is a fast way to plot altitude corrections. I hope at some point to plot the entire path altitude corrected (which will result in everything lining up, but the shadow bath wiggling along) but this is not how it works now. Instead OWC plots the non-altitude corrected path - the green, blue and red lines of the path. When you pick a loction to observe from, OWC will query online the altitude for this location and will apply altitude corrections. Then it will plot the while line that shows you where your actual altitude corrected chord is positioned after the corrections. To read this, you simply need to look at where is the white line drawn relative to the blue, green and red lines of the non-corrected path.
In your case the white line tells you, your actual cord is very close to the edge of the shadow. OW Desktop's plot only shows you the position of the altitude corrected chord and again it is very close to the edge of the shadow.
Now looking at the GoogleMap on OWC you can see that the altitude correction pushes your cord UP and away from the center line. If you want to have a station and a chord that is close to the center line, what you need to do is to select a location on the map that is DOWN from your current position, about 0.7 half path widths. This will place you close to the center and using the white line and an indicator you will be abel to confirm exactly where this cord will be located. You can move it a few time until you are happy with the resulting altitude corrected chord.
Of course if this is a fixed observatory you can't move your chord, and in this case you just need to know that your fixed observatory location is close to the edge of the shadow, rather than beeing deep inside the shadow, as it appears in the non-altitude corrected path on GoogleMaps.
I hope this makes sense.
Hristo.
On Sunday, April 21, 2024 at 09:10:35 AM GMT+10, V Sempronio < vastronomy@...> wrote:
(166404) 2002 NP41 occults TYC 5061-00646-1 OW announced (black highlighted event) that a change had occurred to the above event. I selected it and noticed my selected? observing location is very near the outer edge of the shadow path.  I then looked at the event in OWC and sure enough, there was an update on the 19th. the OWC event maps matches what I see when I invoke the OW "save google earth KLM" feature, and my observing location on both show up half way between the center line and the edge of the shadow path. The path graph in both OW and OWC show my position at the edge of the shadow path. What is going on here? Is it an altitude issue? Also, what is the white line and why does it point from my observing point to somewhere (arbitrarily?) on the white line?  An finally,? what do I, as an observer need to do to make everything line up correctly? Vince
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