Close-range vertical photography for measuring cover changes in perennial grasslands

• Published in: Journal of range management 2000-11, Vol.53 (6), p.634-641
• Main Authors: Bennett, L.T, Judd, T.S, Adams, M.A
• Format: Article
• Language: English
• Subjects: accuracy; Biomass; Cameras; Estimate reliability; estimation; Estimation methods; fire effects; fires; Grasses; grassland condition; ground cover; ground photogrammetry; image analysis; Image classification; Measurement/Sampling; Photography; Pixels; Point estimators; semiarid grasslands; Themeda triandra
• ISSN: 0022-409X; 2162-2728
• DOI: 10.2307/4003159

We describe a method of close-range vertical photography and digital image analysis for measuring changes in total projective cover in perennial tussock grasslands of semi-arid Australia. Repeated photographs of permanent plots (1 m2) were classified using supervised image analysis, providing a clear and objective record of the effects of single-burns on grass cover relative to controls. Computer simulations of the photographic set-up indicated that errors due to camera perspective were consistently less than 4% across a range of cover classes. Other errors, including misclassification error, were not quantified because simplified laboratory tasks indicated that conventional field methods, such as point sampling and visual estimation, provided unreliable estimates of grass cover and were therefore not suitable benchmarks for assessing the photographic method. However, the presented data indicate that the photographic method was sufficiently accurate and precise to measure treatment effects over time and to elucidate relationships between independent growth parameters across a range of cover conditions. In addition, the photographic method was inexpensive, involved minimal field time, and utilised commercial software to classify images. It has the clear advantage over more traditional methods of providing outputs that are readily archived for retrospective studies. /// Describimos un método de fotografáa vertical de rango estrecho y de an·lisis de imagen digital para medir los cambios en la cobertura total proyectiva de pastizales perennes amacollados de la región semiáida de Australia. Fotografáas repetidas de parcelas permanentes (1 m2) se clasificaron utilizando el an·lisis de imagen supervisada, proveyendo un registro claro y objetivo de los efectos de la quema simple en la cobertura de los zacates en relación a las parcelas control. Simulaciones computarizadas de las fotografáas indicaron que los errores debidos a la perspectiva de la camara fueron consistentemente menores al 4% a través del rango de clases de cobertura. Otros errores, incluyendo el error de clasificación equivoca, no se cuantificaron porque las técnica simplificadas de laboratorio indicaron que los métodos convencionales de campo, tales como el muestreo de puntos y la estimación visual, proporcionaron estimaciones poco confiables de la cobertura de los zacates, y por lo tanto, no fueron una referencia confiable para evaluar el método fotogr·fico. Sin embargo, los datos que se presenta