Key takeaways
- Mid-December gives an Edmonton-latitude site roughly seven and a half hours between sunrise and sunset, against roughly seventeen in late June. The usable flying window inside that is shorter again.
- Low sun is the bigger problem, not short days. At Edmonton's latitude the sun reaches about thirteen degrees above the horizon at noon on the solstice, so every object on site casts a shadow several times its own height, all day.
- Measurement deliverables survive winter light better than visual ones. A surface or a stockpile volume is usually fine. Reading surface condition, cracking, staining or small defects out of a shadowed orthomosaic is not.
- Snow cover is a separate problem from daylight, and it changes what a surface means rather than how long you have to capture it.
- The field day is not the daylight day. Travel in light, ground control, site orientation and cold battery handling all come out of the same seven hours.
- Plan one site per winter day in most of the province, and expect a multi-site mobilization that worked in July to need two days in December.
- Some winter work is better than summer work. Frozen ground reaches muskeg, soft leases and ditch bottoms that a summer crew cannot walk, and bare deciduous canopy exposes ground that is hidden in August.
A site visit booked for the second week of December is not a shorter version of the same visit in June. It is a different job with a different budget of light, and the schedule that worked in summer will not fit. The most common winter scheduling mistake in Alberta is not booking a day that is too cold. It is booking a day that is too short.
Cold has an operating envelope, and it is covered separately in winter drone operations in Alberta. This page is about the other winter constraint, which is daylight: how much of it there is, how low the sun sits inside it, and how much work honestly fits between the two ends of it.
How much daylight there actually is
Alberta sits far enough north that the seasonal swing is severe. At the latitude of Edmonton, around 53.5 degrees north, the solstice in late December gives roughly seven and a half hours between sunrise and sunset. Late June at the same spot gives roughly seventeen. Calgary, about two and a half degrees further south, gains about twenty minutes on the December figure. Grande Prairie loses about half an hour against Edmonton, and Fort McMurray, near 56.7 degrees north, is down to roughly six and a half hours.
That is the gross figure, and the usable window inside it is shorter. The first and last part of the day is spent with the sun on the horizon, where the light is too weak and too oblique for a camera-based capture. In practice a December field day in central Alberta offers something closer to five or six hours in which a photogrammetric flight is worth launching, and that window is fixed. You cannot start earlier and you cannot run late.
The flat-light compensation that works in summer does not exist here either. A summer crew that loses the middle of the day to wind can wait and fly at six in the evening. A December crew that loses two hours to wind or fog has lost a third of the day's capacity.
The sun never gets high, and that is the real problem
Short days are an inconvenience. Low sun changes the imagery. At solar noon on the December solstice the sun's maximum elevation above the horizon is roughly 90 degrees minus the latitude minus 23.4 degrees. At Edmonton's latitude that is about thirteen degrees. At Calgary's it is about sixteen, at Fort McMurray's about ten. For comparison, the same calculation in late June puts the Edmonton sun at about sixty degrees.
Shadow length is the tangent of that angle working against you. At sixty degrees of elevation an object casts a shadow a little over half its height. At thirteen degrees it casts a shadow more than four times its height. A ten metre light standard that throws a six metre shadow in June throws a forty metre shadow at noon in December, and it throws a longer one before and after noon. There is no hour of a December day in Alberta when the sun is high. Shadow is the condition of the whole day rather than something you schedule around.
What that does to each deliverable is uneven, and worth being specific about:
- Surfaces and volumes usually survive it. Photogrammetry matches texture between overlapping images, and a shadowed surface still has texture. Stockpile volumes, terrain surfaces and contours from a well-flown winter capture are normally sound, provided the pile is not snow covered, which is a different issue addressed in surveying stockpiles and sites under snow.
- Visual condition assessment degrades badly. If the point of the flight is to read cracking, spalling, staining, corrosion, ponding or small surface defects, a shadow four times the height of every object on site will hide a meaningful share of what you are paying to see. On an orthomosaic the shadows are baked in, and the reader cannot move them. What an ortho can and cannot be used for is set out in how to read an orthomosaic.
- Anything vertical shadows everything near it. Tanks, buildings, berms, treelines, pit walls and conveyor structures all cast a long shadow onto the ground beside them, which is frequently the ground the client cares about.
- Solar thermal work is effectively out. A radiometric thermal inspection of a solar array depends on solar loading to produce the temperature difference that reveals a fault, and a December sun at thirteen degrees through a cold, often hazy sky does not deliver it. The conditions that make a thermal survey interpretable are listed in the conditions that make or break a thermal inspection.
- Building envelope and roof thermal work gets better. The same season that kills solar thermal helps envelope thermal, because the inside-to-outside temperature difference across a heated building is at its largest. Winter is the right time to fly a heated roof, and the wrong time to fly a solar farm.
The field day is not the daylight day
Seven hours of daylight is not seven hours of flying. In a single winter field day the same window has to absorb the drive in and out, which many clients want done in daylight on winter highways; the site orientation and sign-in, which on an industrial site can be an hour before anybody is allowed in the field; ground control placement and survey, which is slower in snow because the points have to be found, cleared and sometimes marked twice; and the battery handling that cold demands, where packs are warmed before a flight and cycled through a vehicle between flights.
Ground control is the step most often underestimated. Placing and surveying targets on frozen, snow-covered ground takes longer than placing them on bare dirt, and the targets have to stay visible from the air against snow, which usually means a different target than the one used in July. How many points a job needs in the first place is covered in how many ground control points you actually need.
Add those up and the honest planning figure for a December day in central Alberta is one site, with the flights sequenced so that the deliverable the client needs most is captured first. If the day falls apart at two o'clock, the thing that matters is already on the card.
What fits into one winter day
A realistic December day, flown by one crew, covers roughly:
- One yard, pit, pad or facility, captured for surfaces and volumes, with obliques for the record.
- One heated building or small group of buildings for envelope thermal, which pairs well with the above because it does not need high sun.
- Either of those plus a short confirmatory re-fly of something captured earlier in the year, if the sites are close together.
What does not fit is the summer pattern of three or four small sites in a day across a hundred kilometres of road. If a multi-site mobilization has to happen in winter, price and schedule it as two days rather than one, and be explicit with the client about which sites are flown on which day.
What to move to spring, and what should not move
Push to spring: anything whose whole purpose is reading fine surface condition from imagery, any solar thermal work, any job where the deliverable depends on measuring bare ground that is currently under snow, and any large multi-site program where daylight is the binding constraint on cost.
Keep in winter, or deliberately choose winter:
- Year-end inventory. A stockpile count tied to a fiscal year end has a date that cannot move. Fly it in winter, state the snow condition in the method note, and accept that the measurement carries that caveat. Reconciling that number against the scale house is covered in drone stockpile volume versus scale-house tonnage.
- Anywhere access is the problem rather than light. Frozen ground carries a crew and a ground control survey into muskeg, soft leases, ditch bottoms and pit floors that are impassable in summer. For some sites winter is the only practical window, and the short day is the price of getting there at all.
- Sites under deciduous canopy. Bare branches let a camera see ground that is invisible in August. The sun is low, but the trees are transparent.
- Heated-structure thermal. As above, the temperature differential is a winter asset.
What this means when you book
Three practical consequences. First, book earlier. A winter job has fewer hours per day and fewer workable days, because wind, blowing snow and fog take days out of the calendar, and the limits that ground a flight are set out in wind limits on a drone survey. A date two weeks out in July is a date three or four weeks out in December.
Second, state the deliverable priority in the scope, not on the day. If the volume matters more than the imagery, say so in writing, so the crew sequences the day to protect it.
Third, expect the method note to carry the conditions. A capture flown at a sun elevation of twelve degrees over partial snow cover is a legitimate dataset, but it is a dataset with properties, and those properties belong in the record so that next year's comparison is made with them in view. Which conditions were in force on the day is part of what makes a capture defensible later.