Using TPE, Part 4: the Horizon

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Here’s the fourth in a series of tutorials on The Photographer’s Ephemeris.

We covered the basics of using the program in Part 1. In Part 2, we covered the Twilight information and the Details View (most of it at least). In Part 3 we covered the use of the secondary map marker. You’ll need to have understood the material in those tutorials before tackling this one.

This tutorial is based on Beta 0.9.6. Click on any screenshot for a full-size expanded view.

The Horizon

Why should the photographer care about the horizon? Simply put, it’s the visible boundary above which the sun or moon rises and below which they set. Knowing where that boundary lies can be important for setting up your shots.

It’s common experience that you can see farther when stood atop a mountain, a tall building or when flying in an aircraft. The distance to the visible horizon increases in proportion to your height above the ground. If you can see farther, then you’ll see the rising sun sooner than if you were back on the ground, or the setting sun later.

So, in short, height above the horizon changes the precise times of sun/moon rise/set.

TPE can adjust for height above the horizon. In this tutorial we’ll walk through the steps to accomplish that.

It’s optional

One important note: this is all optional. You don’t have to worry about it. By default, the times for rise/set that TPE gives match those of the vast majority of other online sources. Very few of these correct for height above the horizon, and you likely won’t run into many problems as a result.

The usual advice applies: arrive at your location early and be prepared to stay late. Do that, and the differences in rise/set times due to height above the horizon most likely won’t worry you.

So when does this matter?

So when might you want to worry about it? Here are some example situations:

  • Shooting sunrise from a mountain peak (e.g. looking east across the San Juans from the summit of Mt. Sneffels)
  • Shooting last light striking a mountain peak (take your pick of summits)
  • Shooting a seascape from a high sea cliff at sunrise/sunset (when and where will the sun set as seen from the 601m high Slieve League in Donegal, Ireland?)
  • You need to know how far you might be able to see from a high point on the landscape (e.g. can I see Shiprock, New Mexico from Mesa Verde, Colorado?)

Let’s discover how.

Back to the Rockies

We’re going to return to the Dream Lake location we used in Part 1.

Tutorial Part 4 Step 1

  1. The quickest way to get back there (assuming you didn’t already save it as a Location) is to search for “Tyndall Gorge, Colorado”
  2. This should drop you just a little east of Dream Lake
  3. Set the date to September 12 2009

Next, let’s position ourselves a bit better for where we’re planning to shoot from:

Tutorial Part 4 Step 2

  1. Drag and drop the primary map marker (red) to a point at the east end of Dream Lake, then pan and zoom to show the top of Hallett Peak and Flattop Mountain to the west

It’s going to be a sunrise shot, so let’s get our time set correctly:

Tutorial Part 4 Step 3

  1. Click Details to switch display modes
  2. Click Next event to move to sunrise and then drag the slider a couple of minutes later to 06:46 hrs
  3. Drag and drop the secondary marker (grey) to a point on the eastern flank of Hallett Peak, aiming for the most tightly packed contour lines
  4. Note that the apparent altitude from lakeshore to mountain flank is 20.4°

So far so good.

But what we are really shooting?

But let’s think about it. What are we shooting here? Where will the rising sun fall?


Hallett Peak from Dream Lake

Hallett Peak from Dream Lake (this was taken in March rather than September, but the rising sun is at a similar azimuth)

Really, we should have our primary marker on the mountain sides: that’s where first light will strike – not the ground underneath our tripod. We need to reverse the marker positions. Fortunately, there’s an easy way to do that:

Tutorial Part 4 Step 4

  1. Click the Swap button next to the Geodetics label (or press the S key)
  2. Note that the two markers swap positions

The point we’re photographing is significantly higher than the lake – some 2,110 feet higher. If you’ve ever been to Rocky Mountain National Park, you’d likely have noticed that you can see clear to the east for a rather long way. That’s because the plains lie several thousand feet lower in elevation. Let’s find out exactly how much lower:

Tutorial Part 4 Step 5

  1. Zoom out so you can see the plains of eastern Colorado as shown
  2. Reposition the secondary marker along the sunrise azimuth line, dropping it somewhere beyond Interstate 25
  3. The Geodetics panel tells us that the change is elevation is some 7,000 feet and the distance over 40 miles

Setting the Elevation at the Horizon

This is the critical step.

Knowing that the plains are just that – plains, and therefore flat – we can use our roughly positioned secondary marker to set the elevation at the horizon. TPE, knowing the elevation at both primary and secondary marker positions, can then take the difference to calculate the elevation above the horizon, which is the number we need to adjust the rise and set times:

Tutorial Part 4 Step 6

  1. Click Lock in the Elevation at the Horizon panel: this locks the elevation at the horizon to the secondary marker position (you can also manually type a value into the text box if you prefer – press Enter when you’re done). Note that the elevation above sea level at the secondary marker position is displayed in the text box.
  2. Note that the time of sunrise has changed: it’s now 06:33 rather than 06:42, and the azimuth has changed also. That nine minute difference is the effect of the elevation above the horizon when up on the flank of Hallett Peak
  3. Finally, note that the azimuth lines are now split either side of the secondary marker – although our time of day setting hasn’t changed, now that we’re accounting for height above the horizon, sunrise is sooner and occurs farther north

You may be wondering, just because we happened to drop the secondary marker just east of I25, that doesn’t mean that’s where the visible horizon is, right? Right. It’s an estimate. Remember the trial-end-error elements of Tutorial 3? This is another one of those. However, the program gives us a clue as to how close we might be:

Tutorial Part 4 Step 7

  1. Zoom out a little (click on the zoom control, press Ctrl-minus, use the mouse scroll wheel) to reveal…
  2. A visual indication of the implied distance to the horizon
  3. If you hover the mouse over the horizon indicator icon, the tooltip will indicate the calculated distance to the horizon

So, if the elevation above the horizon is the 7,000 ft implied by our marker locations, we should be able to see around 111 miles to the east (from the flank of Hallett Peak). Note that the distance is an estimate, based on theoretical (but reasonable) calculations and assumes a ‘standard’ set of atmospheric conditions. See Andrew Young’s Distance to the Horizon page for more details – there’s some interesting background material here too.

Having zoomed out and seen that the implied distance to the horizon is much farther than the location we selected to determine Elevation at the horizon, it makes sense to adjust and double check:

Tutorial Part 4 Step 8

  1. Drag the secondary marker to the limit of the visible horizon and release
  2. You’ll see that elevation above sea level is slightly lower again at this location, and the distance to the visible horizon increases slightly, but not significantly

Finally, let’s test a little further away and see if we can tease the horizon even further out:

Tutorial Part 4 Step 9

  1. Drag the secondary marker a little further along the sunrise azimuth line
  2. The change in elevation is minor (the plains are getting pretty flat our here) and the horizon limit doesn’t follow us. We’re beyond the visible horizon and a warning icon is displayed next to the Distance and bearing label in the Geodetics panel

This time, we’ve gone too far. The secondary marker is beyond the calculated distance to the horizon. We can move it back in and call it done.

What have we achieved with all this?

Let’s review what we’ve accomplished:

  • We’re shooting sunrise on some mountain peaks that lie to the west of an extensive plain at lower elevation
  • We know that the sun will be seen from the mountain peaks earlier than it would at a lower elevation because the distance to the horizon is greater
  • If we want to correct the rise and set times for this “dip of the horizon”, we need to tell TPE what the elevation above sea level is at the horizon
  • Adopting a simple trial and error approach, we can drop the secondary marker in a likely looking location, Lock the elevation at the horizon to the secondary marker position and let the program recalculate
  • By zooming out we can see the implied distance to the horizon and use that as a hint of where to try the secondary marker next
  • With a little trial and error, we can get a decent estimate of where the visible horizon will lie
  • If you were shooting from the mountain peak (as opposed to shooting the peak itself) the distance to the horizon will show you what landscape features you might see in your shot

Gotchas

The same gotchas apply as from Tutorial 3 – you need elevation above sea level for both marker positions. However, in addition:

  • The distance to the horizon will vary depending on which direction you look in. Therefore, it’s important to establish the horizon in the direction from which the light is coming or in which you plan to shoot. (For example, above, the distance to the horizon in the east is very different to the distance to the horizon to the west.)
  • You need to pay attention to the contour information contained in the topographic map in order to make educated trial and error attempts
  • In varied terrain, you may need to test more locations than you might in flatter terrain as used in this example
  • If you need to establish the elevation at the horizon, but still wish to use the secondary marker for other purposes (e.g. as per Tutorial 3), then do the following: (i) establish the elevation at the horizon first, using the Lock function; (ii) once set, Unlock from the secondary marker – this leaves the elevation at the horizon set, but you can now move the secondary marker freely without changing it
  • If you need to the clear the elevation at the horizon, click the X button to the right of the Elevation at the horizon text box

If you’ve enjoyed this tutorial, you might also enjoy “Understanding Light with The Photographer’s Ephemeris” co-authored with renowned landscape photographer Bruce Percy. It’s available through Bruce’s web-site

[Originally posted on stephentrainor.com on 20 Aug 2009 · 21:47:17]