Here’s the third 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 a little deeper into Photo Ephemeris Web’s functionality as well as looking at twilight information and shadows. You’ll need to have understood the material in those tutorials before tackling this one.
You might also be interested to review Technical Note: Sightline Analysis
Geodesy? Geodetics? What’s that all about? I’ll admit that until I started really getting into writing TPE, I didn’t have a clue. However, it turns out that it is a very useful thing for a landscape photographer to know.
I’ll leave it to Wikipedia to explain the details, but in essence, geodesy deals with the measurement and mathematical representation of the earth.
The earth is round, sort of. In fact, it’s sufficiently not round that measuring point-to-point distances on the surface of the earth is only poorly approximated by assuming a sphere. You wouldn’t want your airline pilot navigating this way!
An ellipsoid is a much better model to use, but the math gets hard – so hard, in fact, that a decent solution for calculating point-to-point distances between points on the surface of an ellipsoid was only devised in 1975 by Thaddeus Vincenty.
The geodetics panel and Photo Ephemeris Web’s secondary map marker (the grey pin) use Vincenty’s algorithms to enable functionality that will help you plan shoots in great detail.
Let’s make something absolutely clear here: rises and sets are defined as when the upper part of the sun/moon rises/sets above/below the HORIZON. You may have higher terrain between you and the horizon. That hill behind you is NOT the horizon. To work out when YOU will see or lose the sun or moon behind that hill we use geodesy.
Before we begin
To access the tools used in this tutorial, you'll need ensure you're signed into your account. We'll be looking at the basic point-to-point geodetics tool, which is available free to all users, and also the sightline analysis tool, which requires a PRO subscription. You can decide which makes most sense for how you use the app.
Our destination for this tutorial: the Macey Lakes
Colorado’s Sangre de Cristo Wilderness contains some of the most spectacular peaks in the whole of the Rockies. There are around 18 drainages within the wilderness boundaries, many with stunning alpine lakes surrounded by jagged mountainous cirques.
Let’s find our location.
Note: That while the area is referred to as "Macey Lakes," the search function yields no results when searching for this phrase, so for this tutorial we will search using the place name "Macey Lake" instead. Check out Search gives no results for tips on troubleshooting your searches.
Press Search above the map, then type 'Macey Lake' or ‘38.000339, -105.571876’ into the field, click search or hit return, then click Go to position the primary pin and return to the map:
- Type into the search field
We are going to use a topographic view for the map. If necessary, click the map controls button at the top left of the map and then choose "OpenCycleMap Topo".
The primary map marker (the red pin) should be positioned over Lower Macey Lake.
For the purposes of this tutorial, set your date to July 5, 2025 by clicking the date controls and selecting or typing the date:
Let’s set up our shot. Drag and drop the red pin to the northeast shore of this lower lake (the one farthest to the northeast of the group of three).
Now click the geodetics button (the grey pin button) on the right of the map, or use the keyboard shortcut: G. Two things happen i) the grey pin appears to the east of the red pin and ii) the geodetics panel appears at the bottom of the map.
Meet your new friends!
- The geodetics button shows or hides the grey pin
- The secondary grey pin appears to the east of the red pin the first time you enable it
- The geodetics panel appears along the bottom edge of the map. This shows some numerical data along the bottom, and, for free users, a preview of the sightline analysis tool above. PRO users will see the sightline analysis - we'll discuss that more later
Using that grey pin is what this tutorial is all about - we call it the secondary pin.
A few things about the secondary pin and the geodetics panel:
- It’s optional – you don’t have to use it at all if you don’t want to. Just click on the grey pin button again to dismiss it
- It is “joined” to the primary pin by a grey line, which indicates the bearing from primary to secondary
- By default, it will always appear to the eastern side of the map the first time you use it. If you dismiss and reapply it, the grey pin will appear at the position you last set it, unless it is outside the bounds of the map, in which case it will default to the due east position once again. You may have to zoom out to see both pins if you have moved the red pin.
- Moving it won’t change your sun/moon rise/set/phase or twilight times (at least, not by default – but read see Using Photo Ephemeris Web, Part 4: Horizon).
- The geodetics panel shows information from the primary to the secondary pin: distance, bearing, change in elevation, altitude. You can compare the apparent altitude (between the primary & secondary pins) to the altitude of the sun and moon displayed in the chart legend for the selected time.
We won’t learn much by leaving the grey pin alone, so let’s see what useful information it can provide us.
When will I lose direct sunlight on Lower Macey Lake?
Looking at the map, you can see that the Sun will set to the northwest at this time of year (the dark orange azimuth line). It’s also easy to make out the high ridge line in the same direction. The highest point of the ridge is Little Baldy Mountain. Just eyeballing the contour lines, it seems likely that the Sun will disappear behind the ridge well before it actually sets below the true horizon (see What is sunrise?).
But when? We can use the secondary pin to find out.
Start by looking at the geodetics panel and note the altitude: this is the apparent altitude angle from the red pin to the grey. By convention, we use 'elevation' when talking about a height (feet, meters) and 'altitude' when talking about an angle of elevation (degrees). 'Apparent altitude' means the elevation angle, corrected for the curvature of the Earth and the effects of atmospheric refraction, which allows to see slightly more distant objects than we otherwise would.
Now, drag and drop the grey pin on the summit of Little Baldy to the west (see the image below for the position). You’ll notice that when you do, the geodetics information in the geodetics panel changes, most significantly for our purposes, the altitude from the red pin to the grey is now +18.79°. Yours may be a little different: remember the reading is dependent on the exact placement of the map pins.
What does this number tell us? As mentioned, the data displayed in the geodetics panel is referenced in terms of travel from the red pin position to the grey pin position. So let’s look at the geodetics panel information from left to right in more detail:
- Elevation Offsets: the +5 ft value is the elevation offset applied to the red pin - the typical height of a camera on a tripod. There's no elevation offset applied to the grey pin. PRO subscribers can adjust these values for use cases such as drone photography (height above the ground) or building alignments (the height of a building in your shot). See Using Photo Ephemeris Web, Part 10: Shooting Buildings and Other Objects
- Distance: distance is the shortest point-to-point distance along a great circle from the red pin to the grey pin. The distance from the red to the grey pin is 4,302 ft - the distance accounts for any change in elevation along the sightline.
- Azimuth: the map bearing from the red pin to the grey pin in degrees (note: this is relative to true north, not magnetic north – the same applies to all azimuths and bearings, unless you enable the magnetic declination correction, available to PRO subscribers). The bearing from the red to the grey pin, is 282.74°
- Δ El: elevation refers to height above mean sea level. The change in elevation is measured from the red pin to the grey pin. The change in elevation from the red to the grey pin is +1386 ft
- Altitude: the units of degrees and the use of a + or – give away that this is altitude in the astronomical sense. If you had a sextant and took a sighting to the peak from the red pin position, this is the angle you would measure. This is an ‘apparent’ value, meaning that the measurement is adjusted for refraction i.e. the bending of light caused by passage through the atmosphere. The apparent altitude from the red to the grey pin is +18.79°
Note: the altitude is not exactly what you’d get by dividing the elevation change by the distance and calculating the inverse tangent: a simple 'flat earth' calculation does not account for the curvature of the Earth’s surface or effects of refraction. The difference is small, but increases with greater distance.
Now we know what we’re looking at, let’s find out the altitude of the Sun when it passes through the same bearing at the peak of Little Baldy, where the grey pin is positioned.
Start by estimating when you think the Sun will disappear behind the ridge - just pick a time, say around 6:15 p.m., and adjust the time using the time slider.
You can advance the time slider in ten second increments by clicking on the time slider 'thumb' then using the keyboard left and right cursor keys. By advancing the time slider bit-by-bit, the Sun azimuth line will align with the pin bearing line - and therefore with the summit of Little Baldy - at around 6:27 p.m.
But will it be visible from our red pin position?
We know the peak of Little Baldy lies at +18.79° from the red pin. Looking at the Sun’s altitude in the geodetics panel, you can see that it lies at +21.33°, a couple of degrees higher than the summit of Little Baldy.
So, the Sun will still be visible at 6:27 p.m. from our spot on the shore of Lower Macey Lake, assuming the ridge line is indeed the blocking feature in the sightline.
We need to look a little further to find out exactly when we will lose the Sun from our red pin position.
Let’s start by moving the time slider a little later to 6:40 p.m. The Sun’s azimuth moves around closer to the sunset azimuth. Look at its apparent altitude: it is lower in the sky. Drag the grey pin a little farther to the northeast along the ridge line to sit on top of the Sun’s azimuth line again. Note your apparent altitude from the red pin to the grey. The Sun’s altitude is around +18.78° and the geodetics apparent altitude is +18.77° - the Sun is just setting behind the ridge.
Note: the apparent altitude of the Sun is measured for the center of the disc. So, at 6:40pm, the top 'half' of the Sun only remains visible.
You’ll need to apply some judgment here and look at the contours of the topographic map (it’s difficult to do using other non-topographic map types) and see where the sensible test points should be. We’ll look at this in more detail below.
Note: Slight variations in the positions of the pins yield slightly different results. Always be prepared and arrive at your shooting destination early!
So far, so good. However, you've probably noticed that moving the pins back and forth requires a little trial and error work. What if there were a better way? That's where sightline analysis comes in. You'll need to be PRO subscriber to use this tool. With a PRO subscription, here's what you'll see at 6:40pm for this shot setup:
A few things to point out in this chart:
- The sightline goes from the red pin (left) to the grey pin (right), irrespective of the relative position of the pins on the map. Imagine you're photographing from the red pin, shooting towards the grey pin.
- The red pin is always shown as displayed above. The grey pin can be shown either dark or light, depending on whether it is visible from the red pin. In this case, it is not visible, which makes sense, as you can see from the contour lines on the map, that the pin is positioned slightly beyond the top of the ridge line
- The solid dark grey line indicates that the corresponding point on the map is visible. Dotted line indicates that the point is not visible as viewed from the red pin. (The light grey solid lines are marginal: they might be visible, but we can't be certain, based on the accuracy limits of the digital elevation model that we use.)
- The orange line shows the altitude (elevation angle) of the Sun from the red pin. Looking closely, it looks like it might actually be obstructed, and our 6:40pm time might be slightly too late.
It doesn't matter so much where you drop the grey pin
One of the key advantages of the sightline is that, for the purpose of establishing what can and cannot be seen, the exact end point of the grey pin doesn't matter so much. Let's drop a way further out, well beyond the ridge line:
The grey pin is now out at the far shore of the lake over the ridge to the west. The sightline shows that nothing beyond the ridge is visible from the red pin, as you'd expect.
I've hovered the mouse over the elevation profile at a point 1.12 miles along - look at the chart legend: it shows the distance, elevation and altitude, plus a 'not visible' icon (an eye crossed out). The corresponding point on the map is highlighted with a target icon, so you can easily relate points along the sightline to features on the map.
It's now even clearer that the Sun is likely already set at 6:40pm - why? Likely because we didn't quite identify the point with the greatest apparent altitude when we did our trial and error approach above. Adjusting the time back a couple of minutes to 6:35pm and moving the grey pin to the south to re-align with the Sun's azimuth, we get:
That looks better.
With sightline analysis, it's much easier to identify "topographic rise and set" times with accuracy. The risk of missing the right test positions with the grey is largely eliminated.
Note: if you're on a smaller screen, you may wish the reduce the vertical height of the sightline chart. Click the down arrow at the right to collapse the chart, or the up arrow to expand it once again:
Will the rising Sun strike Point 13,200’?
Now we’ll look at a different question. Zoom out one step on the map.
Let’s say you want to make a sunrise image of Upper Macey Lake (the larger lake to the southwest of the red pin position) and you’d like to take in the cirque to the south of the lake. However, the image will likely only work if the top of the cirque catches the rising Sun. You can use Photo Ephemeris Web to determine if the rising sun will be obstructed or not:
- Upper Macey Lake
- The cirque surrounding the lake to the south: we are trying to find out if the Sun will illuminate the southwest of the cirque at sunrise
First click on the sunrise event in the timeline, the time slider and legend jump to 5:46 a.m.
Now that we have located our position, you can zoom in again on the map. If the red pin is now outside your map area just hit the center red pin button to move it to the center of the map or use the keyboard shortcut: C
Move the red pin to the top of the peak near the contour label 4,020 m (or 13,200 ft) on the map. (Note: contour lines and units will depend on which map type you have selected. In the screenshots below, I'm using OpenTopoMap.)
Engage the geodetics function again by either clicking the grey pin button on the right of the map, or use the keyboard shortcut 'G'. If your grey pin is still in the old position this will turn the function off, just engage it again to place the pin to the east of the red pin in the visible map. Now place the grey pin along the sunrise azimuth line on the first ridge line to the northeast.
Notice the elevation angle and change of elevation figures: that ridge line sits below our marked peak. We don't need the sightline analysis to draw the clear conclusion that the ridge will not obstruct the rising Sun.
- Clicking the sunrise event in the timeline moves the time slider and legend to that moment
- The grey pin is set on the first ridge we can see along the sunrise azimuth line
- The elevation of the ridge line to the northeast is 664 ft below the red pin position
- The apparent altitude from the red pin to the grey is -12.48°
So far, so good: the first ridge line lies below our peak by some margin, so we should get some direct light. However, to be sure, let’s check to see if Colony Baldy, that large mountain to the northeast, will cause us any problems.
Zoom out two levels on the map (zoom controls at the bottom right). Move the grey pin out along the sunrise azimuth line again and drop it on the eastern flank of Colony Baldy:
Here, the sightline comes into its own: it's obvious that the Sun is not obstructed anywhere along the path. You can also check this using just the free geodetics tool, but you'll need to be careful to sample enough points along the sunrise azimuth line to be sure you've covered all the potentially trouble spots - you can see from the sightline that are a quite a few close calls.
Good news. We should be able to make the shot. Now that we know our rugged mountain ridge will also receive some direct light, we can hope for a decent photograph.
Can we really see the ridge line?
In this example you’ll see why elevation angle and apparent altitude are so important.
Let’s say we want to determine the angle of view to the ridge line in the cirque to the west of the upper lake. Will the ridge line actually be visible from the lake? This would be good to know before we set out on that exhausting hike to “Upper” Macey Lake.
Let’s move the red pin to where we plan to shoot from by the lake. Now move the grey pin to the ridge line south west of the lake, opposite where the sun will rise.
It might be that we’ll be looking at a false summit in front of the ridge line as seen from our position on the lake shore. Can we determine if that's the case?
To find a position opposite sunrise click the sunrise event in the timeline. The time slider and legend should now be at 5:46 a.m. Click on the time slider then advance the time by 3 minutes. As you adjust the slider, the sunrise extension line is displayed under the sun shadow: the line extends through the red pin and continues to the south west. When you release the mouse from the time slider, the shadow lines are hidden. Hold the Shift key down to show the extension lines, then move the grey pin to where the Sun extension line crosses the ridge line (if you have trouble with this, try starting to drag the grey pin first and only then press and hold the Shift key).
The apparent altitude from the red pin to the grey is +22.00°.
- The red pin is set on the north east shore of Upper Macey Lake
- Clicking the sunrise event in the timeline moves the time slider and the legend to that moment
- Use the time slider to advance the time by 3 minutes
- The grey pin is placed on the Sun azimuth extension line where it meets the ridge line above the lake
With the sightline, it's plain to see that some areas are obstructed and not visible from Upper Macey Lake.
Firstly, there's something obstructing the view of the lake itself - it appear to be some higher ground between the red pin and the lake shore:
Let's move a little closer to the lake shore. That's better:
However, the top of the ridge line is still out of sight:
In this instance the forward thrusting buttresses of the cirque wall probably won’t impact our images significantly, but it’s important to be on the look out for these details in some situations. We can see most of the way without obstructions.
Using only the secondary pin
If you're not a PRO subscriber you can still spot check the sightline. Again, this is where some trial and error and map reading skills are involved. Let’s test it by moving the grey pin down the slope a little to where the contours are packed a little tighter.
Note the increased apparent altitude, it’s now +23.97°. This means that while the elevation may not be as great as the position at the top of the ridge line, the apparent altitude from the red pin to the grey pin position here is steeper by a couple of degrees - hence, the true ridge is obstructed.
The Geodetics calculation can determine distance and bearing quite happily just from the map marker positions (which we always know by definition – you placed the markers). However, to do anything more, we need to know the elevation above sea level for both marker positions. Some potential gotchas:
- Photo Ephemeris Web uses a mix of elevation data from SRTM1, SRTM3, AsterGDEM and GTOPO30 and other sources.
- The underlying elevation data points are usually spaced either every 30 or 90 metres (1 or 3 arc-seconds). Relying on this for high precision, short distance work is not recommended - you should conduct a site survey.
That said, for most landscape photography uses, this will work well. However, if you have a once-in-a-lifetime shot that requires critical planning, I recommend that you:
- Consult multiple reliable sources for Sun/Moon information (I highly recommend Jeff Conrad’s Sun/Moon Calculator – Jeff has kindly provided invaluable feedback and guidance for Photo Ephemeris over the years).
- Obtain a large-scale topographic map of the area of your shoot from a reputable publisher and take careful measurements of distance and elevation.
- Consult the online tools from the National Geodetic Survey and perform your own geodetic calculations.
- Scout the location in advance, if possible. In this case, that was tough ask - for us it was a four hour drive, followed by a seven mile hike with 2,500 feet of elevation gain!
The next tutorial will cover Elevation at the horizon. If you’re shooting from high places, to want to catch first light on a prominent mountain summit, this can be important: Using Photo Ephemeris Web, Part 4: the Horizon