Looking Down into the Night Sky

Astronomy is one of my longest running scientific hobbies. Growing up in the suburbs of Minneapolis I got to witness light pollution firsthand, one half of the night sky too washed out to see anything. This problem followed me down to Winona for school, working at the campus rooftop observatory and having to block out the lights below with my arm just to see anything.

When the opportunity came up to do an undergraduate research project studying light pollution in town I jumped at it. A recent convert to the world of GIS, I immediately saw this as a spatial problem, and that I should map the brightness around town. I got to combine my dual interests in astronomy and geospatial science. Bonus!

This began with a task of driving around parts of town at night with a handheld light meter, a specially designed light pollution meter, and a Trimble GPS to both record the position and the meter values. Some problems with this started to show up, as light pollution levels varied on a large scale (some areas were brighter than others) and the small scale (I avoided standing too close to street lights, but at what point is that altering the survey?).

During this time I was taking a field methods class and was flipping through the class guidebook (Comptons Field Methods), and I came across a section I never considered before: aerial surveying methods. Thus began my adventure into aerial light pollution surveying.

The concept is simple: light pollution is largely artificial light directed up into the night sky, reflecting off of air and moisture and washing out celestial objects. So to measure light pollution, go to where the light is going: fly above the city and look down. The light you are seeing downward is, in a sense, light pollution. Satellite imagery and light readings have been taken of many parts of the world at night but not at the resolution needed to pinpoint specific contributing sources within cities.

I had been working at the airport for a little while so using one of the aircraft was little problem. Location (and even elevation data) would be recorded with Trimbles in the aircraft. The Cessna had a small porthole in the rear floor which could be opened (for aerial photography).

The handheld light meter would sit in the porthole with a fixture I made. This would effectively tell us the "brightness" of the belly of the plane (the double layering of the floor gave an aperture effect so that the light meter was telling us brightness directly downward and not so much from the sides). But the main light measuring instrument was something most people have: a digital SLR camera.

A foam sleeve would fit around the camera lens and keep it from falling through the porthole. Exposure and focus settings were manually set so pictures would be consistant.

The next tricky bit involved a lot of funky math (Excel is my best friend sometimes): what elevation should this nighttime aerial survey be flown at? A number of factories had to be considered. Flying too high would mean more climbing time, and the plane isn't cheap. At a higher altitude, the resolution of ground features would also be less. Flying high would also mean fewer passes, as each picture would cover a larger area, and fewer passes meant less flying time. But because it would be at night, the shutter speed was slower than normal, and flying too low would cause the lights to show up as streaks from a longer exposure time. Flying too low would mean a lot of passes, and a lot of pictures, with the possibility of gaps occuring in the continuum of photos.

By some wizardry I don't care to explain I came to an optimum value: 6000 feet above ground. This was actually the cheapest elevation to fly (a balance between climbing time, descent time, and time at elevation) to cover the entire city with the least amount of photos without sacrificing resolution etc etc. The next bit was to figure out what camera settings would produce the best image at this elevation. To do this I went up to a scenic overlook of the city and played with settings to see which would provide the best image of the lights that were about a mile away.

By knowing our elevation, the area each photo would cover, and the aircraft speed, I knew I had to take a picture every 15 seconds to ensure some overlap of the photos along each line. Three passes would cover the whole city with a little extra and, this part happened to be very coincidential but I'm going to claim otherwise, by making standard rate turns at cruising speed each turn-back would cause the next line of photos to overlap the edges the previous line by just the right amount (about 15%), agan ensuring no gaps. Just like the diagram in Compton's! It looked like I knew what I was doing.

The date was set for the night flight. We would want to do the survey during "typical astronoy hours", about an hour after sunset to 11pm. Does it matter? It might. Some businesses may shut off their lights after a certain time, and I wanted data to best reflect astronomy hours.

Equipment was ready, the flight was planned from startup to shutdown, and we had 3 Trimbles ready (just in case). One of my flight instructors would be piloting, one of my professors would be ensuring GPS-itude, and I would be sitting in the back of the plane taking the photos and measuring the light through the portal.

This was quite a strange ride sitting on the floor in the back of a small plane at night. I couldn't see outside at all. To begin the survey we would pass over the airport at 6000 above ground going eastbound. In the meantime I opened up the portal and prepared the camera setup. Through the LED viewer on the camera I could see a few streetlights, but it wasn't much to see. Every few seconds I would snap a new picture, and at the end of the line I was instructed we would be making the first turn. I took this chance to take the camera out and take a look for myself.


Looking straight down at a city at night from 6000 feet through a foot-whide hole in the bottom of a plane is something that doesn't happen very often. I will never forget it.

Two more passes were made. A slight wind was giving us a bit of a drift which my professor caught by watching the GPS track on the Trimble, which we were able to correct for. Afterward, I stuck the light meter into the ground and would call out brightness values every few seconds, which would be recorded with a time value.


What good is time, though? Fortunately, the Trimble time-stamped each GPS data point, so time and position were interchangeable. Also, the digital camera time-stamped each photo, so the position of each photo could also be quickly determined. To make sure the camera and the Trimble were synced to the same time I just took a picture of the time on the Trimble display (clever, no?).


My next task was putting this plethora of information into ArcGIS. The light meter data was easy, just a few data points with lux values. The aerial photography was a big trickier, and I spent many many hours georeferencing aerial photographs, which showed up quite well (individual streetlights, even ones next to eachother, are distinguishable).

Fortunately, street lights are easily referenced to streets, and larger buildings and parking lots could be referenced to regular satellite imagery. The end result of the photography was a georeferenced, high resolution image of Winona at night.

The important thing about the georeferencing is that I could compare both the light meter readings from the airplane and the readings from the ground. We only managed two passes using the light meter but it actually did pick up areas which were brighter than others.

The large "bright" area in the middle is the group of lights just right of center on the aerial photo, and the second smaller "bright" area is from the smaller group of lights down and to the right. This whole operation took about two hours of flying time, which was great, as ground surveying (while higher resolution) was very time intensive. By combining the information obtained with the aerial and ground surveys we were able to pinpoint specific locations in the city that were contributing most to light pollution. Specifically, businesses and commercial zoning districts were the brightest areas in the city, while only comprising a small percentage of the area. Traditional "save the night sky" advice is typically aimed at proper residential lighting but from this it seems like light pollution action should be focused at businesses.

Stay in Formation

One idea that I always played aroung with was that of going on a flight with someone else. Not just with me in the plane, but in another plane, sort of as a formation. Who could blame me? One of my greatest disappointments in life is that I would never be a member of Rogue Squadron (I have admitted numerous times to friends that I have logged more hours in an X-wing than an actual airplane, without question).

So the next best thing would be to fly around with a friend, just to see what the hype is all about. Luckily at the small airport I called home I was training at the same time as a good friend of mine, and each of us were specializing in one of the two training planes.

My friend really liked the Cessna...

And I preferred the Piper...

Identical in nearly every aspect of performance (as far as a student pilot is concerned), sometimes you just like one style over the other (the Piper felt "lower" and more stable on the ground to me). The main difference is just where the wings are positioned, and that results in other differences which a pilot might prefer or dislike. For the Piper, the wings are underneath. This means you have a large blind spot underneath you, and no shade from the wings (so it gets hot in there in the summer, but stays warmer in the winter). The opposite is then true for the Cessna (blind spot above you, but better view below. The shade of the wings keeps you cool in the summer, not as warm in the winter). Also, the Piper needs pumps to get the fuel in the wings to the engine, while the Cessna is gravity fed (the wings being above the engine). As someone that fueled these little planes for a living, I would also add that Pipers are easier to fuel, since you need a ladder or good balance to get up to the tanks on the Cessna. The minute details go on forever, but I digress...

After deciding this flight must happen, the next step was of course making sure we could. Would we be violating any rules? Is it safe? Etc, etc. To do this, of course, we went to the latest edition of the FAR/AIM (the hefty little book that lists all the rules pilots must follow), to see what we could do, and it had this helpful information:

FAR Sec. 91.111

Operating near other aircraft.

(a) No person may operate an aircraft so close to another aircraft as to create a collision hazard.
(b) No person may operate an aircraft in formation flight except by arrangement with the pilot in command of each aircraft in the formation.
(c) No person may operate an aircraft, carrying passengers for hire, in formation flight.

One of the many joys of learning to fly is coming across aviation regulations which are intentionally ambiguous, so that judgements may be made more or less subjectively (flying 10 feet away from an aircraft may not create a collision hazard for the Blue Angels, but definitely for us). This, then, meant that it was up to our discretion to determine what would create a collision hazard, so we satisfied (a). We were discussing it with eachother, so no problem with (b), and we were not doing anything for hire so no (c). Now we just had to get away with it! (It might be legal, but they aren't our airplanes).

We picked a nice, mostly-clear day to go (actually 4 years ago this day). It was somewhat hazy, but we didn't mind. Visibility was still a number of miles.

Another non-flying friend of mine came with me to take some pictures and video, while two of my other flying friends took up the Cessna to do the same. We agreed on a rendesvouz point, and a radio frequency to use which wouldn't interferece with anyone else. Because of the haze we didn't climb up too far (the ground started to disappear above a few thousand feet). After a few minutes of back and forth on the radio we finally saw them, lazily heading eastward.

We pulled up next to them about 100 feet away or so, just to see how we felt about it (pulling up next to someone in a vehicle moving in 3 dimensons is a little tricky). After a minute of getting situated, the other airplane decided to give us a wave.

We went back and forth over the whole area, trading places and basically just playing follow the leader, chatting back and forth on the radio and generally pretending like we were in Top Gun (without the fancy maneuvering or weaponry...).

Throughout the whole flight the best pictures definitely came from the other aircraft, as we were mostly taking video (which I will put up here eventually). Some of the photos turned out quite well.

It was a great time for all of us. Despite the fact that we never caused any trouble, and at best were only close enough to see the other aviators wave back at us, I couldn't help but feel like we had done something semi-rebellious. Nevertheless, it was a good experience, and definitely something I needed to get out of my system. Though...throughout the flight I couldn't help but look down at the Mississippi River Valley...

 ...and dare it to have a thermal exhaust port less than 2 meters wide.

Epi(c)karst!

During the middle of my research on the karst geology in Springfield my field site was blessed with a visit by the highway department expanding a nearby intersection. All geologists enjoy road cuts. Especially fresh road cuts (seen as the fresh red soil in the aerial photo).

Springfield is atop a very shallow karst system. Soil is usually very thin (meters or less) and made of reddish-brown clay. This grades down into the Burlington-Keokuk Limestone, a nearly-white to gray coarse-grained limestone that is extremely crinoidal (these crinoids are usually more resistant to weathering and will stick out of weathered surfaces).

Limestones in general are susceptible to dissolution, and the Burlington is no exception. When water gets into fractures in the rock these become widened and form caves. When these enlarged conduits drain soil from the surface or collapse entirely they form sinkholes. All of these processes are occuring underneath the city, and due to the complex nature, are nearly unpredictable...dealing with sinkholes is largely a remediational rather than a preventional plan.

Rainwater percolating through this supper soil layer has the highest dissolution potency right when it hits the bedrock. Because of this, the uppermost layer of the bedrock, at the boundary with the soil, undergoes a very aggressive level of dissolution. This produces a very irregular bedrock/soil boundary known as epikarst. This boundary gives engineers a lot of grief, especially when they need to drill down into the bedrock and install structural supports. This produces a system of pinnacles (limestone sticking up into the soil layer) and cutters (soil columns descending down into the bedrock layer).

While down in the newly scoured section I had a chance to take some great pictures to explain the general geology of my field site.

Clay-rich soil (brown to dark red) and broken limestone blocks (white).

Irregular soil (dark red)/bedrock (white/brown) boundary. Notice how the white chert nodules stick out of the limestone as the chert does not dissolve while acting as a flow barrier which causes enhanced dissolution at its edges. The brown mottling on the limestone is typical of the weathered surface, and a great place to find crinoid stems.

 

Sections were blasted down into the bedrock exposing the more cohesive limestone. The nearly continuous horizontal white bands are layers of chert, common in this lithology.

Two photos showing the complete picture: a thin (1-2 meter) soil layer (transitioning from the upper, light tan soil to the deeper, dark red soil), down through the epikarst boundary (irregular soil/limestone boundary), down into the limestone (here still weathered and fracturing). The water is ponded from construction and not represeting the water table here (although saying that this unit even contains a water table is debatable).

Amidst the exploring we also discovered some rather large (~2 inches across) brachiopod fossils in the limestone.

 

I have not seen brachiopods in the Burlington yet, but literature does indicate it is in the Pierson Formation below (if you skip through the Elsey to be cherty lower member of the Burlington). Either they had cut down into the other bedrock units or they happened to blast down into a facies of the Burlington that contains the brach's. Either way, great samples, and a nice chance to look into the near-surface karst geology.