LARGE SCALE SAMPLING PHOTOGRAPHS
FOR FOREST SURVEYS IN CANADA
By L. Sayn-Wittgenstein
Research Officer, Forest Inventories Section, Department of Forestry, Ottawa
Abstract The purpose for which large scale sampling photographs may be employed is
described; past investigations and accomplishments are reviewed; the information that can
at present be obtained from such photographs is dealt with. Current investigations and the
most important remaining problems are described.
Résumé Les buts de l’utilisation de photographies aériennes à grande échelle de régions
modèles font l’objet d’une description; les recherches et les résultats antérieurs sont passés en
revue; on traite les renseignements que l’on peut actuellement obtenir de telles photographies.
Les recherches actuelles et les problèmes en suspens les plus importants sont mentionnés et
décrits.
Zusammenfassung Es wird der Zweck der Verwendung grossmasstäblicher Luftbilder
beschrieben eine; Übersicht über frühere Forschungen und ihre Ergebnisse wird ebenfalls
gegeben; der gegenwärtige Stand der durch dergleichen Aufnahmen verliehenen Auskunft
wird behandelt. Gegenwärtige Forschung sowie die wichtigsten noch zu lösenden Probleme
werden beschrieben.
“Sampling photographs” correspond to the sample plots of a ground survey
and the information obtained from them is roughly the same as that collected
from sample plots, viz. detailed measurements of individual trees and the
description and classification of the forest.
In contrast to general-coverage photographs, which involve the photo
graphing of the entire area under investigation, sampling photographs are
taken of only a small portion of that area.
To meet the need for exact and detailed information, sampling photographs
are taken at large scales, usually between 1 : 500 to 1 : 2500. But, because
only a small portion of an area under investigation has to be photographed,
cost of large-scale photography seems justifiable.
In the past, work with large-scale photographs has encountered the follow
ing two main problems, only the first of which has been overcome:
1. Photographs were not sharp enough for exact measurements, largely due
to the effect of image motion.
2. The scale of photography could not be accurately determined and reliable
measurements were therefore impossible except where accurate ground
control was available.
There have been many attempts to solve the first of these two problems by
using fast shutter speeds, panning cameras, or the image-motion-compensating
Sonne cameras. The last method resulted in extremely sharp pictures but it
had little value for measurements because scale could not be controlled.
Today, the problem of image sharpness has been adequately solved because
