Carta Exploration Ltd

Feasibility and Permitting — See the pattern of retreat of Alberta’s Athabasca Glacier over the past 100 years from vintage ground photographs and air photos.

Location
Athabasca Glacier, Rocky Mountains, Alberta, Canada

Importance
This case study shows how to measure change on air photos using an independent reference point to determine photo scale.  This technique is vital when dealing with measurements of active processes of change that modify the landscape.   

The length of the glacier and thickness of ice at the toe has changed dramatically since 1906 (Figs. 1, 2).  Luckman (1999) estimates Athabasca Glacier receded over 1200 m from 1906 to 1999, Sunwapta Lake formed in 1940 and calving of the glacier into the lake continued until 1968. 

Problem and Objective
Measure the amount of glacial retreat in the available air photos and topographic map to calculate the average rate of retreat over the interval. 

Air Photograph Interpretation
A series of air photographs dating from 1939 to 1993 (reproduced in the figures) show the retreat of the glacier away from the road and towards its source.  Careful examination of the air photo records determines the timeline of recession and related geomorphic change. 

How to Measure Distances from Air Photos
The following is the sequence of steps to measure the amount of glacial retreat over time.  The numbers used in the calculations are from actual measurement taken from the printed figures.

Step 1  Calculate the reference distance
a)       The first step is to print the figures (Figs. 3-6) individually on letter-sized paper.  The scale of all images will have to be determined because it likely will vary from any stated nominal scale.

b)       Choose a reference distance between two distinctive topographic or map features that does not change over time and is identifiable on all images.  For example, the distance, Q, between the highway and the creek draining the southeast lateral moraine (Fig. 4) where it flows free of the confining moraine marked by a distinctive bend in its course.  Other two points will work but watch for highway realignments, creek bed wandering and other sources of subtle change between photographs, which will throw your calculations off.

c)       Calculate the scale of the topographic map (Fig. 6) by measuring the map distance between blue grid lines, which are 1 km = 1000 m = 100,000 cm apart.  Map distance is 1.45 cm.  Use the scale equation, RF = map distance / ground distance, yielding: RF = 1.45 cm / 100,000 cm = 0.0000145.  Invert to obtain the ratio 1:68 965 for the scale of the topographic map. 

d)       Calculate the reference ground distance Q.  To do this, use a ruler to measure Q on the topographic map.  Suppose Q = 1.2 cm.  Ground distance = map distance / RF = 1.2 cm / 0.0000145 = 82,758 cm.  Thus the reference distance Q is 828 m long.

Step 2  Use reference distance to calculate the scale of the air photos
a)       Determine the scale of each printed photo.  To do this, locate the reference distance on the air photo.  In this example, look for the sharp bend of the creek as it clears the tip of the sharp-crested lateral moraine.  You may need to view the scene stereoscopically in order to find the reference points.  Use a ruler to measure between the shortest distance from creek bend to the highway.

b)       Suppose, you measured on the 1939 photograph #43, the reference distance to be 2.25 cm.  Using the scale equation, RF = 3.25 cm / 82758 cm = 0.0000392.  Invert to obtain 25,464.  Thus the scale of the 1939 air photo is 1: 25 464.

c)       Do the same measurements and calculations for the other editions of photographs.

Step 3  Calculate the distance between glacier snout and highway for each edition of photography
a)       Use the shortest distance between glacier snout and the highway as the reference distance to determine glacial retreat.  Other reference points will work too.

b)       On the 1939 air photos, a photo distance of 2.25 cm was measured between glacier and highway.  Rearrange the scale equation to get: ground distance = map distance / RF = 2.25 cm / 0.0000392 = 57,398 cm.  Thus the distance between glacier and highway is 574 m.

c)       Repeat the calculation for the other air photos.

Step 4  Calculate the amount of retreat over time
From 1939 to 1958, the glacier moved 424 m over a span of 19 years giving an average retreat speed of 22 m per year. 

From 1958 to 1993, the glacier moved 325 m over a span of 35 years, yielding an average retreat speed of 325 m / 35 years = 9.3 m per year. 

From 1939 to 1993, the glacier retreated by 1323 m - 574 m = 749 m over a span of 54 years. 

The overall average rate of retreat is calculated as 749 m / 54 years = 13.9 m per year, as determined by air photo interpretation.