Friday, July 4, 2014

Answer: Why could I run on water?

Here's the map from a recent run... and the questions.... 

Can you help me figure out what happened here? 

1.  How is it possible that, while running on the beach, I seem to have run on the water?  What is a non-supernatural explanation for this? 
2.  What was my speed while running this course?  (In either kph or mph, I'm happy with either.)  
3.  While running in the park I noticed that the ground I was running on was not quite what I expected.  What kind of ground was I running on? 
4.  (Extra credit)  Here's a picture of something I ran past at the end of Lincoln Way, just off the end of the road.  What is it?  (No, I know what the windmill is.  What I'd like to figure out is what's the blocky thing with all of the graffiti on it.  What's that?  Although the story behind why there's a windmill at this particular location is interesting as well.)  

This first one is easy.  If you search for 

     [ Google Map Engine ] 

you'll find the Map Engine (at ).  This is a tool Google makes that lets you create your own map.  

Using this, I tried to duplicate the map from above.  Here's my (slightly simplified) map.  

Note that you can change the "base map" by clicking on the down-arrow (to the right of the Base map menu item).  Here I change it to "satellite view" (the second item in the top row of options).  

and now when I look at the map (slightly zoomed in on running-on-water stretch), I can see what happened... 

My path took me out onto the sand... at low tide.  The edge of the water is marked on the map at "mean high water"  (sometimes called "average high tide") since that's the more-or-less conventional place to mark the edge of the sea on maps.  

So it's not that hard.  Just use the right tool, and look at the different views.  

Now in truth, we got lucky.  The image on the satellite view HAPPENED to be at low tide.  

Here, just for comparison, I put the Bing satellite view and the Google satellite view top/bottom.  

If I'd only looked at the Bing map, I'd still be wondering about my water-walking abilities.  Note that it's not that Google satellite view is better, it's just that the Google image happens to be at low tide. (Actually,  FWIW, the Bing map shows the park trails in more detail than the Google one does.)  

So it's useful to know about multiple, different versions of the same data (or imagery).  Contrasts between them often reveal useful differences.  

If you create a close-enough copy of my running route using Google Maps Engine, you'll see a path editor, which will ALSO give you the distance.  

From this it's easy to see that the total distance is 3.89 miles.  Since this took me 38 minutes to run, I used Google's conversion like this: 

or I could have asked for it in kph, but this was just handy.  

If you've been reading the blog for a while, you'll know that I run a fair bit.  (I'm running a 5K later this morning, and have run 7 marathons in my time. I love to run the trails in the Santa Cruz mountains, where, truthfully, many of the ideas for this blog have originated.) 

So... why so slow?  

Let's figure out what the ground surface is made of.  How would we do that?  

One way would be to read the original garden design and see what the plan was.  There's a nice summary in the Wikipedia article on Golden Gate Park, which points out that in 1860s, "Golden Gate Park was carved out of unpromising sand and shore dunes that were known as the Outside Lands, in an unincorporated area west of San Francisco’s then-current borders."  

The best image I could find (by searching for [ archival images Golden Gate Park ] ) is this one that illustrates what the park looked like BEFORE all of the planting.  I did this to validate what I'd read about the park being made over sand dunes.  Looks like those claims are correct.

This is from the north end of the park area (Seal Rocks) looking south towards the area that would become Golden Gate Park.  As you can tell, it's all sand as far as the eye can see.  

This is the substrate on which Golden Gate Park is formed... it's sand everywhere, and the sand comes up into the trails, which is what I was running over.  

I know what surface I was running on, but for you to find out, StreetView is a pretty handy way to get the actual images from the places where the trail crosses city streets.  Here are a few images: 

The sand is obvious where the trail crosses the Great America Highway (near the windmill)--it's ALL sand there.  But even at 25th Street (where I started), it's all rocky (which was put there to stabilize the sand), and at the Sunset Blvd crossing, you can see it's all sand there under the shade.  

But let me tell you, running on loose, dry sand is tiring...and slow!  (I really can run faster than 6.14 mph.)  

Now, the last thing.  

Here are a few images from Google Maps: 

Earth view from straight overhead.  Note the large pool around the structure.

Same location, but this time in an Earth view, tipped and rotated to see the front. Note the grate. 

From a nearby photosphere.  Note the grate.  

Another view of the structure.  Someone has changed the background color on "Surfer."  

To figure out what this was, I searched for: 

     [ sewer Ocean Beach San Francisco ] 

(Why add in "San Francisco"?  Because there are many "Ocean Beaches" around the world.  Apparently all of them have sewage issues, so this gives me links to only the one I really care about.)  

And in there I found a hyperlocal newspaper:  The "Ocean Beach Bulletin" which published this diagram in an article about beach closings.  This diagram labels our mystery structure as the "Ocean Beach (North) combined sewer discharge."  

This gave me an excellent search clue:  

     [ sewer discharge Ocean Beach San Francisco ] 

which led me almost immediately to the SPUR (San Francisco Planning and Urban Research) report on a plan to manage Ocean Beach's combined sewage and storm drainage.  Report link.  In there I found this fascinating tidbit (p. III-8): 
"In wet weather, stormwater runoff surges into the system. When
the plant’s capacity of 65 million gallons per day is overwhelmed,
the transport box and Lake Merced Tunnel — two massive
structures designed to store runoff and prevent overflows — fill up
and retain the combined flow. Overflow there is decanted and
pumped to the deep ocean outfall. Only when that system’s
capacity is exceeded do combined discharges occur, through two
large outfall structures on Ocean Beach

There you have it: the structure (with its large grate, and periodic appearance of a pool in front of it) is an overflow valve for the combined sewer/storm drainage system on the western side of the city.  The good news is that this happens rarely, but when it does, the Ocean Beach is closed to swimming and surfing.  The bad news is that the beaches close because the overflow is running onto the beach.  

Search lessons:   

As we've discovered before, having the right tool really helps.  Learn about Google Maps Engine--it's incredibly handy (and we only touched the surface of what it can do).  

Second, when you want to figure out what something in the real world is like, nothing beats going and looking for yourself.  If you can't get there, then tools like StreetView, Maps, and Earth are your best friends.  You can actually LOOK at something, and not have to take someone else's word for it. 

Third, as we found out from the running-on-water episode, Maps have built-in assumptions.  In this case, the assumption is that the "water line on the map is the high water mark," and not the low water mark... 

Finally, searching for "hyperlocal" news sources can often solve your hyperlocal question, or at least provide a great resource that gives you additional search terminology to refine your search.  

Search on!  

(And don't run on the beach immediately after a heavy rainstorm.  You might get more than you bargained for...) 

1 comment:

  1. Another fun question/answer/search-skill tip.

    Wednesday night or Thursday morning I submitted an answer to #3 regarding the area's pre-park sand dunes (and that loose sand was likely to have been your "unexpected" running surface ... but it disappeared after I was asked to log in to Google account (which I did). I thought it had still been submitted and was awaiting moderation,as the comment box was blank after I'd logged in ..which is the comment-submission behavior I see at other blogs/articles. But since it still hasn't shown up as a comment to Wednesday's post, I surmise now that it was deleted instead. I hope that user-experience flaw can be fixed, somehow.