How do you find a Needle in a haystack? A map might help.

Mapping weeds at a fine scale helps us plan ecological restoration. Many grasslands have a mosaic of different weed infestations- from dense swards to scattered individuals. Each demands a different control approach. Mapping can help select targets, allocate resources and evaluate works, reducing the time a crew-member spends on foot lugging equipment across site. However, such mapping is time-consuming so is rarely used for routine projects.

How can we make weed mapping more affordable?

High definition aerial photography that is frequently updated could make mapping of some environmental weeds more economical. In Melbourne, Nearmap™ [1] provided access to aerial imagery at low or no-cost options for non-commercial applications from 2009 until late 2012 when commercial rates were enforced.  Just how useful could mapping derived from these photos be? Could it justify paying the commercial rates?

One of the most serious environmental weeds threatening native grassland in the Merri Creek valley is Gorse. Gorse was originally introduced as a hedging plant and soil stabiliser and while useful, it is also invasive and competes with threatened grassland vegetation. It is highly flammable, increasing fuel loads and making ecological burns more complex to conduct.

This investigation, funded by Caring for Our Country, attempted to interpret Nearmap™ images to map Gorse cover at Kalkallo Common grassland before being treated. Gorse’s dark colour stands out against grasses- even in aerial photos.

A secondary aim of the investigation was to develop a procedure for training student volunteers to gather the data needed to ground-truth the aerial photo maps. Students from land management courses can provide ‘free’ labour but it is a challenge to generate accurate assessments with only a brief instruction, while also providing students with a worthwhile learning experience. Our exercise to assess the photo interpretation and also create maps for directing Gorse control works was conducted in 2011 with Holmesglen TAFE students. The students proved adept at concurrently assessing the seriously weedy Toowoomba Canary Grass. The exercise was repeated in 2013, after control works, with Epping NMIT students. Refinements between the first and second exercise improved efficiency.

Our work shows strong potential for aerial photo interpretation for Gorse mapping. Within an hour, an experienced observer could produce a map that was similar to one produced by a dozen students over three hours. However, aerial photo analysis is unable to detect very young and sparse gorse, a serious shortcoming as these are often the highest priority to treat. The student exercise generated accurate maps and was popular with students and teachers of Conservation and Land Management.

This new evidence will help us and other grassland managers to consider the costs and benefits of paying for aerial photography. However, we can already say that the student mapping exercise is useful for both MCMC and the students and will be included in annual planning.  

The report is available at MCMC website Information/Publications section under Restoration projects or can be read here.