Thursday, August 10, 2017

BRC, Javascript + SVG

I'm creating a map of Black Rock City in SVG using javascript, so that I can map some of the locational Burning Man API data I've been playing with. It took me quite a lot of time to get to flow on this project. It's not quite finished, but the latest version is here.

It took some thought to translate GPS data with distances in feet, into SVG's xy-origin-at-upper-left coordinates. To make sure that everything is as it should be, for now, real GPS coordinates correspond to actual xy coordinates on the map.  e.g. "the golden spike" at 40.78660,-119.20660 in GPS is represented in the SVG graphic as <circle cx="-119.2066" cy="40.7866" r="0.0005" fill="gold"></circle>. Note how the coordinates are identical.

The problem with this is that it's necessary to zoom way, way in, because the distances involved are tiny. For example, the distance from "the golden spike" to one of the pentagon corners is 8,175 feet, but in "GPS units", it's only 0.0293. Just a single foot is 0.000003! The width and height  of the whole thing is only 0.7 x 0.6 (in SVG's unit-less distances). Such tiny numbers makes dealing with fonts difficult. But it is pretty neat, to just be able to grab a coordinate and stick it in without modification, so I'll experiment before deciding to scale it up.

Adding to my confusion is that the official GPS coordinates[pdf] for the enclosing pentagon appear to be way off (or my mind is way off). I assumed it was my transformations somehow, which is why I took extra care to make sure that there was a one-to-one correspondence between GPS coordinates and a spot on the SVG map.  There are now zero translations or transformations of any coordinates, but the official coordinates make a sad and collapsing pentagon (here shown in blue).  Way off, right?  It looks like it was crushed!

The really odd thing is that the coordinates match  Google maps almost exactly. If you enter those coordinates into Google maps, the marker will land almost precisely on the pentagon corners of last year's satellite photo of Black Rock City.  Strange.

If you have any ideas about what happened, here, let me know! I'm still baffled.

Edit:  When I calculate the distances to the Man from each of these official pentagon corners, I get these values for each, which are consistent with the visuals. They should all be equal, and they should all be more-or-less 8175. As you can see, they vary from a few hundred feet to over a thousand.

BM Official Pentagon Coordinates
GPS coordinates Distance to Man
Latitude Longitude GPS unit feet

The mystery deepens, but so does my confidence that it's the data and not me!

By contrast, these are the GPS values I'm using, generated by calculating 5 points 72° apart on a 8175 foot radius from the golden spike coordinate. Interestingly, one of the points almost corresponds. The order's not the same, because I generated them widdershins from high noon.

My "fuck it!" coordinates
GPS coordinates Distance to Man
Latitude Longitude GPS unit feet

Again, you can see the two pentagons here: