University of Oulu

Kulha, N., Pasanen, L., & Aakala, T. (2018). How to Calibrate Historical Aerial Photographs: A Change Analysis of Naturally Dynamic Boreal Forest Landscapes. Forests, 9(10), 631. doi:10.3390/f9100631

How to calibrate historical aerial photographs : a change analysis of naturally dynamic boreal forest landscapes

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Author: Kulha, Niko1; Pasanen, Leena2; Aakala, Tuomas1
Organizations: 1Department of Forest Sciences, University of Helsinki
2Research Unit of Mathematical Sciences, University of Oulu
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 3.3 MB)
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Language: English
Published: Multidisciplinary Digital Publishing Institute, 2018
Publish Date: 2018-11-14


Time series of repeat aerial photographs currently span decades in many regions. However, the lack of calibration data limits their use in forest change analysis. We propose an approach where we combine repeat aerial photography, tree-ring reconstructions, and Bayesian inference to study changes in forests. Using stereopairs of aerial photographs from five boreal forest landscapes, we visually interpreted canopy cover in contiguous 0.1-ha cells at three time points during 1959–2011. We used tree-ring measurements to produce calibration data for the interpretation, and to quantify the bias and error associated with the interpretation. Then, we discerned credible canopy cover changes from the interpretation error noise using Bayesian inference. We underestimated canopy cover using the historical low-quality photographs, and overestimated it using the recent high-quality photographs. Further, due to differences in tree species composition and canopy cover in the cells, the interpretation bias varied between the landscapes. In addition, the random interpretation error varied between and within the landscapes. Due to the varying bias and error, the magnitude of credibly detectable canopy cover change in the 0.1-ha cells depended on the studied time interval and landscape, ranging from −10 to −18 percentage points (decrease), and from +10 to +19 percentage points (increase). Hence, changes occurring at stand scales were detectable, but smaller scale changes could not be separated from the error noise. Besides the abrupt changes, also slow continuous canopy cover changes could be detected with the proposed approach. Given the wide availability of historical aerial photographs, the proposed approach can be applied for forest change analysis in biomes where tree-rings form, while accounting for the bias and error in aerial photo interpretation.

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Series: Forests
ISSN: 1999-4907
ISSN-E: 1999-4907
ISSN-L: 1999-4907
Volume: 9
Issue: 10
Article number: 631
DOI: 10.3390/f9100631
Type of Publication: A1 Journal article – refereed
Field of Science: 4112 Forestry
112 Statistics and probability
Funding: The project was funded by the Academy of Finland (Proj. No. 252629, 276022), Emil Aaltonen Foundation, and the University of Helsinki Funds.
Academy of Finland Grant Number: 252629
Detailed Information: 252629 (Academy of Finland Funding decision)
276022 (Academy of Finland Funding decision)
Copyright information: © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (