Using TPE Desktop Web App, Part 3: Geodetics

Here’s the third 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 went a little deeper into TPE’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.

This tutorial is based on Beta v0.9.7. Click on any screenshot for a full-size expanded view.


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 maths 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 TPE’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.

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:

We are going to use terrain view for the map. If you are not already in terrain view, click “map” and then terrain. (NOTE: Google’s terrain map is not usually available at the very highest zoom levels, if terrain is greyed out and you are unable to click it, just zoom out on the map until you can).

For the purposes of this tutorial, set your date to July 5th 2009. Click the calendar icon, click on the month name at the top of the calendar, click on the year number at the top of the calendar, select 2009, select July, select 05.

Type ‘Macey Lakes, Colorado, USA’ into the search box, and click search or hit return. The primary map marker (the red pin) should be positioned over the Macey Lakes.

Your location should look something like this:


  1. Date set to 5 July 2009
  2. Searching for ‘Macey Lakes, Colorado, USA’ sets the red pin position
  3. Select “map” and check “terrain” to bring up terrain view

Let’s set up our shot. Drag and drop the red pin to the north east of the lower lake (the lake closer to the top of the map). Then centre the map on the new red pin position by holding down shift and clicking the centre red pin button or use the keyboard shortcut: Shift C.

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!


  1. Hold down Shift and then click the centre red pin button to centre the map on the red pin. Or use the keyboard shortcut: Shift C
  2. The geodetics button shows or hides the grey pin
  3. The grey pin appears directly to the east of the red pin the first time you engage it
  4. The geodetics panel appears at the lower left side of the map

Using that grey pin is what this tutorial is all about.

A few things about the grey 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 red pin by a grey line: the pin azimuth line
  • 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 would default to the due east position). 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 – check back for the next tutorial).
  • The geodetics panel shows, by default, the information from the red to the grey pin: distance, change in elevation, azimuth and apparent altitude. The sun and moon information displayed shows the apparent altitude for the sun and moon at the time selected by the time slider at the bottom of the screen. The sun and moon apparent altitude is provided as a quick reference in the geodetics panel so you can compare the values with the red-grey pin apparent altitude.

We won’t learn much by leaving the grey pin alone, so let’s see what useful information it could 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 north west 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 (not named on the map – but see image below). 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. But when?

We can use the grey pin to find out.

Start by looking at the geodetics panel and note the “Alt” number: this is the apparent altitude from the red pin to the grey. My Alt reading is 19.74°. Yours may be a little different: remember the reading is dependent on the exact placement of the map pins.

Now, drag and drop the grey pin on the summit of Little Baldy to the west (see the image below for the postion). You’ll notice that when you do, the geodetics information in the geodetics panel changes, most significantly for our purposes, the apparent altitude from the red pin to the grey is now +19.23°


  1. The sunset azimuth line
  2. Little Baldy mountain is located in the high ridge line to the west of the red pin position
  3. The geodetics panel Alt information is the apparent altitude from the red to the grey pin

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:

  • Distance and bearing: 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 4432ft
  • Change in elevation: 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 +1460ft
  • 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 in v0.9.7). The azimuth from the red to the grey pin, is 282.30°
  • Apparent 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. ‘Apparent’ means that this 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 +19.23°
  • Sun Apparent altitude: the apparent altitude of the sun for the time selected by the time slider. The apparent altitude of the sun is -0.25° (assuming your time slider is set to sunrise, 05:46)
  • Moon Apparent altitude: the apparent altitude of the moon for the time selected by the time slider. The apparent altitude of the moon is -18.70° (again, assuming your time slider is set to sunrise, 05:46)

(Note: the apparent altitude is not exactly what you’d get by dividing the elevation change by the distance and calculating the inverse tangent: the calculation accounts for the curvature of the earth’s surface and adjusts the result for refraction.)


  1. Distance
  2. Change in elevation
  3. Azimuth
  4. Apparent altitude
  5. The apparent altitude of the sun
  6. The apparent altitude of the moon

OK, 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 18:00. Scrub the time slider to 18:00.

You’ll see the sun azimuth line move around during the course of the day. When I get to 18:00 in the time slider, my sun azimuth line lies near to the pin azimuth line.


  1. Use the time slider to select 18:00
  2. The Sun azimuth line moves with the time slider
  3. The pin azimuth line

I can advance the time slider in one minute increments by clicking on the time slider (it highlights blue) then using my keyboard left and right cursor keys. If I advance minute by minute the sun azimuth line will align with the pin azimuth line (and, therefore with the summit of Little Baldy) at around 18:24.

But will it be visible from our red pin postion?

We know the peak of Little Baldy lies at +19.23° from the red pin. Looking at the sun’s altitude in the Details panel, you can see that it lies at +21.94°, that’s +2.71° above the peak of Little Baldy.

So, the sun would still be visible at 18:24 from our spot on the shore of Lower Macey Lake.


  1. The time is now set to 18:24. Clicking on the time slider and then using the keyboard left/right cursor keys allows you to advance the time slider minute by minute
  2. The sun azimuth line is aligned with the pin azimuth line and sits directly on top of Little Baldy
  3. Comparing the apparent altitudes of the grey pin and the sun determines whether the sun is still visible from the red pin position

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 18:37. 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 north-east 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 still slightly greater than the apparent altitude from the red pin to the grey, so the sun is still visible front the red pin position.

By repeating this procedure you can establish that the sun is likely to drop out of sight around 18:39, that’s 108 minutes before sunset at 20:27.

You’ll need to apply some judgement 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.

Ok, here’s an exercise to try if you are feeling supremely confident about using the geodetics function in TPE: find the mountain called Colony Baldy to the southeast of the red pin. Determine how high on the north west flank of the mountain you will observe direct light in the moments just before sunset. Hint 1: you’ll need to relocate both markers. Hint 2: you may need to move the grey pin farther than you think. Answer at the end.


  1. The sun’s apparent altitude sits below the apparent altitude of the red to the grey pin and is therefore out of sight of the red pin position
  2. The time at which the sun will not be seen from the red pin position

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 TPE to determine if the rising sun will be obstructed or not:


  1. Upper Macey Lake
  2. 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 05:46.

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 centre red pin button to move it to the centre of the map or use the keyboard shortcut: C

Move the red pin to the top of the peak near the contour label 13,200’ on the map. 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 north east.

Notice the apparent altitude and change of elevation figures: that ridge line sits below our peak and will not obstruct the rising sun.


  1. Clicking the sunrise event moves the time slider and legend to that moment
  2. Click the centre red pin button to centre the red pin on the map, or use the keyboard shortcut: C
  3. The grey pin is set on the first ridge we can see along the sunrise azimuth
  4. The elevation of the ridgeline to the northeast is 594ft below the red pin position
  5. The apparent altitude from the red pin to the grey is -12.05°

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 one click on the map. Move the grey pin out along the sunrise azimuth line again and drop it on the flank of Colony Baldy.

Note the apparent altitude is still negative (around -3.9°), indicating that the sun will clear Colony Baldy and strike our high ridgeline in the cirque above Upper Macey Lake.

Good news. We should be able to make the shot. We can already see from the basic sun rise line that we should get good light over the lake itself at the moment of sunrise. Now that we know our rugged mountain ridge will also receive some direct light, we can hope for a good shot.

And now for the reason we set this date and selected this location: here is a shot of the lake in question taken on this date!


Upper Macey Lake

Can we really see the ridge line?

In this example you’ll see why apparent altitude is 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 ridgeline 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.

To find a position opposite sunrise click the sunrise event in the timeline. The time slider and legend should now be at 05:46. Click on the time slider (it hightlights blue) then advance the time by 3 minutes using your keyboard cursor keys. 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. However, if you wait a moment before releasing the mouse after you finish adjusting the time, the lines will remain visible. Move the grey pin to where the sun extension line crosses the ridgeline.

The apparent altitude from the red pin to the grey is 21.59°


  1. The red pin is set on the north east shore of Upper Macey Lake
  2. Clicking the sunrise event in the timeline moves the time slider and the legend to that moment
  3. Click the time slider then use your keyboard cursor keys to advance the time by 3 minutes
  4. Holding down the shift key enables the sun azimuth extension line
  5. The grey pin is placed on the sun azimuth extension line where it meets the ridgeline above the lake

OK. But this is where some trial and error (and map reading skills) come in. It might be that we’ll be looking at a false summit in front of the ridgeline as seen from our position on the lake shore.

Let’s test it by moving the grey pin down the slope a little to where the contours appear a little steeper. Note the increased apparent altitude, it’s now +23.78°. This means that while the elevation may not be as great as the position at the top of the ridgeline, the apparent altitude from the red pin to the grey pin position here is steeper by +2.19°.

In this instance the forward thrusting buttresss 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.


  1. Move the grey pin down the slope from the ridgeline to where the contour lines on the map appear closer together
  2. The apparent altitude from the red pin to the grey pin is now +23.78°


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:

  • The TPE for Desktop web app uses Google elevation data
  • The underlying elevation data points are typically spaced every 90 metres (3 arc-seconds) – there may be higher resolution in some parts of the world (particularly the USA, where higher resolution Shuttle Radar Topography Mission data is available), but you shouldn’t rely on it. Google interpolates this to fill in the gaps, so relying on this for high precision, short distance work is not recommended

That said, for most landscape photograpy 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 TPE 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.
  • Maintain your sanguine disposition when, though the clouds cooperated, the sun or moon did not appear quite where or when you expected. Even if all your preparation and calculation was perfect, the vagaries of atmospheric refraction may result in an unexpected outcome.

Answer to the exercise

I make it ~13,300ft. The ridge line of Little Baldy isn’t the limiting factor – you need to look at the next ridge line farther north west which lies even higher. Place the secondary marker there, and then adjust the primary marker up and down the north west flank of Colony Baldy until you obtain an apparent altitude of around zero. From that point upwards, you should see direct light from the setting sun. More or less :)

The next tutorial will cover Elevation at the horizon. If you’re shooting in high places, this could be significant…

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.

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