Where’s the Sun?

It’s a simple question, but one I haven’t found a satisfying answer to: where’s the sun?  How do you look at a map and figure out which areas are mostly sheltered from its rays and which ones receive strong midwinter sun?  When will your campsite first see direct light in the morning, and will it have a good view of the sunset or be smothered in shade hours beforehand?

When I built the viewshed layer, I realized that I could use the elevation data and techniques from it to create all kinds of cool terrain-driven visualizations.  One of the more obvious ones was a sunlight analysis layer that could answer the above questions without the need for napkin math and a protractor.  After a lot of work finding the right balance between accuracy and performance, it’s finally live.

You can add a sunlight analysis layer just like viewsheds and slope shading:

Specify a date, and either pick a specific time or get an average across the entire day:

Click OK and it shows up as a layer checkbox, just like the other custom layers:

Testing these layers is always more fun than building them.  A good opportunity was heading out to the Sierra Club’s Bradley Hut to check on snow conditions for an upcoming overnight trip, coupled with California’s serious drought conditions.  Solar warming isn’t a reliable predictor of snow depth, and my shading doesn’t even factor in vegetation like trees.  Still, it seemed like a good test case.  The route follows the dashed black road in from the red highway, cuts North to a creek crossing, and then continues up the far side of the image below.

The layer uses a blue-yellow-red spectrum, where blue is completely shaded, red represents the maximum amount of sunlight a slope can have on that day, and yellow is 50%.  The coloration is based on how directly the sun shines on a slope, so glancing light counts less than direct, full-on illumination.  It doesn’t account for all factors, but the goal was to provide a rough picture of the sun’s effects, not exact measurements.

The blue areas in the picture below held snow well.  The red-tinted slopes beyond the bridge were melted out (“no snow”) while the yellow spots on the approach were simply thin.  Rough correlation achieved.  Success?  Maybe.

Taking one of the last lifts up KT at Squaw on Friday, the sun was low enough to cast shadows across GS Bowl.  I noted the time, came home and punched it into CalTopo.  The first image shows the terrain with simple sun/shade coloring, the second has the “adjust for intensity” box checked.  The sunlight layer only provides 15 day and 15 minute increments, so I’m not expecting an exact match, but the top of Olympic Lady and upper part of Red Dog Ridge are in the sun, with the lower reaches shaded.  Close enough.

Yellow = direct sun, Blue = shaded

Blue->Yellow->Red spectrum “adjusted for intensity”

But wait, there’s more . . .

Since I’d already done the hard work of computing the sun’s position and tracing along its path, it didn’t take long to throw together detailed year-round information for a specific point.  Right click on a map, choose Point Info and then Sun Exposure.

The dialog below shows day/night and sun/shade information for the current year.  The x axis is time of year, with Jan 1 on the left and Dec 31 on the right.  Y axis is time of day, ranging from earliest sunrise to latest sunset.  Gray = night, Blue = shaded and Yellow = direct sunlight.  Mouse over a point on the graph and you’ll get the corresponding date/time as well as the sun’s azimuth and altitude at that point.

Sun exposure for the Upper Pines campground in Yosemite

Back when the internet was far less mature and AltaVista ruled the search scene, one of the photographs I always wanted to take was of the sun setting across the Golden Gate Bridge.  At the time I could never figure out how to make that happen, except for watching the sun for months on end and waiting for it to align.

Although no longer on my to-do list, the project seemed like a good way to demo integrating sunlight information with other features on CalTopo.  Step 1 is to shoot a bearing from Treasure Island across the Golden Gate:

You can go back and forth with what part of Treasure Island you want to shoot from and where you want the sun to cross the bridge, but I got a bearing of 260 degrees:

Assuming you want the sun to be just over the horizon, the sun exposure table gives me an altitude of 3 degrees at an azimuth of 261 degrees on March 7:

The bridge is 746′ high and Treasure Island is 29000′ away, so the tower tops will be atan(746/29000) = 1.5 degrees above the horizon, so even this is maybe a couple days too late.  However most people would take the photograph from farther South on the island, in which case you’d want to go a little later.

From photographers to backcountry skiers to car campers, everyone can benefit from a good pre-plan.