"Prism" is the canal channel itself, specifically its cross-section. So the prism line is the outline of the prism: the edge of the canal.

"Prism:
The most common element is the canal itself, often referred to as the "prism". The cross-section of this prism is usually trapezoidal in shape. The dimensions of the prism vary from canal to canal. Some early American canals had cross sections 40 feet wide at the water line, 26 feet wide at the bottom, and four feet deep. Later canals were wider and deeper. As needed, the prism was dug into or built up from the surrounding land. If the earth that the prism was built through was not porous, the canal could be just dug. But, in porous soils, the canal had to be lined with a clay puddle. In more recent times, linings can be fabric or concrete."
http://www.americancanals.org/Canal_Structures/Canal Structu...

"Please be advised that the Panama Canal Authority has scheduled maintenance activities at the Miraflores Locks for 12 days, beginning Sunday, May 5th, 2013 at 0001 hours through Saturday, May 11th, 2013 at 2400 hours. Dredging operations will be conducted on the West side of Miraflores Locks between the center wall and the West prism line. During these 12 days, the West Line at Miraflores Locks will be out of service for shipping."
http://www.adimarships.com/panama-canal-locks-lane-outage-5t...

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Note added at 7 hrs (2013-09-16 12:02:26 GMT)
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"Prism line" is definitely the standard expression in English:

"The treaty granted to the United States as if sovereign a canal concession in perpetuity to a canal zone 10 miles wide, 5 miles on either side of the Canal prism line."
http://www.pancanal.com/eng/history/history/american.html

"The southbound sailing line in Balboa Reach has a heading of 160 degrees 35 minutes true, is marked by range markers and lights which are visible day and night, and lies 150 feet from, and parallel to, the west prism line of the Canal channel. The northbound sailing line is the reciprocal heading and is located 150 feet from the east prism line of the 500-foot wide reach."
http://www.leagle.com/decision/19701618311FSupp1307_11364

And so on.