Using Photo Ephemeris Web, Part 4: Horizon

Here’s the fourth in a series of tutorials on Photo Ephemeris Web.

We covered the basics of using the program in Part 1. In Part 2, we went beyond the basics. In Part 3 we covered the use of the secondary map marker (the grey pin) and looked at geodetics. You’ll need to have understood the material in those tutorials before tackling this one.

Prefer a video?

Watch a replay of our "Beyond the Basics" webinar:

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 or the setting sun later than if you were back on the ground.

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 quite advanced, is required only in specific situations, and is entirely optional. By default, the times shown by Photo Ephemeris for rise/set match those of the vast majority of other online sources, where rise/set is stated from ground level to the ideal horizon. Very few of these correct for height above the horizon.

You likely won’t run into many problems as a result of not using this feature. In fact, the usual landscape photographer’s sound 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 won’t concern you.

So when does this matter?

Here are some example situations when knowing about the horizon will help your photography planning:

• 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?)
• You are planning a shot that requires precise alignment between sun/moon and an object in the landscape, and you’re shooting from a point high above the surrounding terrain
• You are shooting from the 33rd floor of a skyscraper, from a drone flying at 1,000 ft, or from an aircraft

Let’s discover how this works in TPE.

Back to the Rockies

Search for 'Dream Lake' or '40.3094, -105.6590' (yes, we’re back in Rocky Mountain National Park). You should see the primary map pin centered on Dream Lake after clicking GO to position the primary pin:

Next, let’s position ourselves for the shot: drag and drop the primary map marker (the red pin) to a point at the east end of Dream Lake. Set the date to September 14 2024 and select Sunrise on the timeline:

We now need to see the top of Hallett Peak (to the west of Dream Lake) as well as the red pin. You may need to zoom out and move the map to get both points visible on your screen.

It’s going to be a sunrise shot, so let’s set our time correctly. Click the sunrise event in the timeline. The time slider and legend will jump to the moment of sunrise for the position of the red pin and the chosen date: 06:43.

Click the grey pin button to engage the geodetics function or use the keyboard shortcut (click on the map first): G. Drag and drop the secondary marker (the grey pin) to a point on the eastern flank of Hallett Peak, aiming for the most tightly packed contour lines

Note that the elevation angle from lakeshore to mountain flank (from primary pin to secondary pin) is +19.23°:

But what we are really shooting?

Perhaps it’s time to think about what we are planning to shoot here. Where will the rising sun fall?

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 red pin on the mountainside: that’s where first light will strike – not the ground under our current tripod position. We need to reverse the pin positions. Fortunately, there’s an easy way to do that:

Click the Swap button on the map or use the keyboard shortcut (click on the map first): S – the red and the grey pins swap positions.

Remember, information in Photo Ephemeris is always expressed for the position of the red pin, and the geodetics information always shows travel from the red pin to the grey pin. Note how the information has changed in the geodetics panel: the change in elevation and altitude numbers are now negative and the sun and moon apparent altitudes have changed slightly to match the new red pin position for the time slider’s selected time.

The point we’re photographing is significantly higher than the lake – over +2000 feet. From Rocky Mountain National Park you can see clear to the east for a rather long way because the plains lie several thousand feet lower in elevation. Let’s find out exactly how much lower.

Zoom out so you can see the plains of eastern Colorado as shown. Move the grey pin out along the sunrise azimuth line, dropping it somewhere beyond Interstate 25/highway 87. Finally, advance the time slider a few minutes to 06:48 so we know the sun is fully above the horizon.

The Geodetics panel tells us that the distance between the pins is over 40 miles and that the change in elevation is more than 7,000 feet. The azimuth from the red to the grey pin is 86°, almost the same as the sunrise azimuth.

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 grey pin to set the elevation at the horizon. Photo Ephemeris knows the elevation at both red and grey pin positions, so can take the difference to calculate the elevation above the horizon, which is the number we need to adjust the rise and set times.

Look at the top of the map towards the right. Click the 'edit' button next to the mountains icon: this allows us to set the elevation at the horizon.

Click the Edit button to show the form:

Once enabled, some additional controls are shown. Check the 'Use geodetics pin elevation'. You will see that the field for the approximate elevation at the horizon is populated (your value may vary slightly from the one shown below, due to small differences in the secondary pin position - they don't really matter):

You can also manually type a value by unchecking the "Use geodetics pin elevation" checkbox, and then typing in your chosen value.

Click Submit.

Now look at the time of sunrise, it has changed to 06:35 rather than 06:43. The sunrise azimuth has changed also, it is now further north of our grey pin – and, 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 to the north.

That eight minute difference is the effect of the elevation above the horizon as observed from high on the flank of Hallett Peak:

The elevation above the horizon is calculated as follows: (Elevation at Primary Pin) - (Elevation at Secondary Pin). The 5 ft difference between what the Geodetics panel shows (7144 ft) vs the elevation above the horizon (7149 ft) is due to the default 5 ft we allow for the camera height above the ground (typical tripod height). (PRO subscribers can adjust these elevation offsets - more on that in a later tutorial.)

You may be wondering, just because we dropped the grey pin somewhere east of the I25 highway, is that where the visible horizon actually is?

Well, no: it’s an estimate.

Remember the trial-end-error elements of Tutorial 3? This is another case where trial and error produces acceptable results. However, the program gives us a clue as to how close we might be.

Zoom out a little on the map (you may need to zoom out 2 or 3 times).

You will see a grey circle centred on the red pin. This is the horizon overlay and is a visual indication of the implied distance to the horizon. The calculated distance to the horizon from the red pin’s position is indicated next to the elevation above the horizon value, above the map. For my red pin position it stands at ~112 miles.

The rise and set azimuths look quite stunted from this far out. TPE limits all azimuths to 200 miles, except for the pin azimuth (they are of little practical use beyond that distance).

1. The calculated distance to the horizon for my red pin position is ~112 miles
2. The horizon overlay is a visual indication of the implied distance to the horizon
3. You can see the ends of the azimuth lines when zoomed out this far

So, from the flank of Hallett Peak, if the elevation above the horizon is over 7,000 ft (implied by our pin locations), we should be able to see ~112 miles to the east. It’s important to understand 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 out along the plain than our first trial and error placement of the grey pin, it makes sense to adjust and double check - this is the trial and error part.

Place the grey pin out along the sunrise azimuth, where it intersects with the visible horizon overlay circle. 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:

What have we achieved with all this?

Let’s review:

• The premise is that we are shooting sunrise on some mountain peaks high above an extensive plain
• 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 grey pin in a likely-looking location, Lock the elevation at the horizon to the grey pin 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 grey pin next
• With a little trial and error, we can get a decent estimate of where the visible horizon will lie
• You can see by now that 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

Buildings, Drones and Planes

You may be wondering, what if I'm high up in a building or flying, rather than on the ground? In that case, you'll need to use Elevation Offsets to account for your height above the ground. This feature is available to PRO subscribers and is covered in a later tutorial: Using Photo Ephemeris Web, Part 10: Shooting Buildings and Other Objects.

Gotchas

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

• The distance to the horizon will vary depending on which direction you look. Therefore, it’s important to establish the horizon in the direction from which the light appears or in which you plan to shoot. (In the example above, the distance to the horizon in the east is very different from the distance to the horizon in 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 horizon overlay locations than the flatter terrain 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, with "Use geodetics pin elevation" enabled; (ii) once set, disable "Use geodetics pin elevation" – this leaves the grey pin free but the elevation at the horizon remains set
• To clear the elevation at the horizon, click on the value in the horizon text box, delete the value and then hit return.

Next up: Using Photo Ephemeris Web, Part 5: Locations