Since you're mostly dealing with digital versions of air phot Hi Jeff, Since you're mostly dealing with digital versions of air photos, (as opposed to histogram matched and calibrated satellite data) you're immediately going to be either hand extracting or "photo-interpreting" the extent of the canopy layer within each of the images or (potentially) using an automated (machine segmentation program) like eCognition, which, depending on a range of factors contained within each image set (lighting, texture, time of year, resolution, parallax, etc....).... Either way, you're best off by having the same person(s) perform the analysis so that areas considered to be "FOREST" can be more uniformly assessed and mapped. (at some point you have to decide "how big does a tree or forested patch have to be, to be considered "FOREST".... with a virtually uniform gradient, spatially as well as temporally, from BARE SOIL to MATURE FOREST, that break line can be tricky to hit.
You'll have to be aware of a number of error factors within the work; how well each year\image epoch was "georeferenced", (or superimposed within a real-world feature space) both to real world features as well as each other... whether elevation factors were included in the correction process (ala orthocorrection processes to remove terrain displacement), etc... All of these can significantly affect the area metrics you compile for each image set. And that's all assuming that you can somehow thread the data of the respective image layers that are served up via Google Earth, into an appropriate image processing or GIS capture software array. In my opinion, you certainly will not want to attempt the exercise within Google Earth itself. While single point, line or polygon feature creation is supported in the software, to attempt to extract large areas of adjacent feature layers would be an extraordinary challenge (it's really not intended to serve as a GIS, but as a simple "geobrowser" or tool to look at things). But if you have access to software like ArcGIS, there very likely may be a way to gain access to the imagery (often in a higher resolution and "clearer" version) as an online feature or map service which NYS and other agencies host. How big is the area you are looking at? There are a number of satellite based end products as well as potential input layers (e.g. Landsat) which, at 30m resolution, you might be able to get some sense of the general trend (there are also versions of certain products that measure change in land cover over time...). -J John Mickelson Geospatial and Ecological Services 501 Stage Rd. Monroe, NY 10950-3217 (845) 893-4110 [email protected] ________________________________ From: "Corbin, Jeffrey D." <[email protected]> To: [email protected] Sent: Monday, July 1, 2013 2:49 PM Subject: [ECOLOG-L] Canopy cover from GoogleEarth images Hello Ecolog – My colleagues and I are studying the reforestation of a reclaimed landfill - from essentially bare-ground to a reasonably dense forest from 1991 to present. I am interested in quantifying changes in percent canopy cover over time using GoogleEarth images. Their archived images include good–resolution growing season photos taken in 1995, 2001, 2005, 2007, and 2010. Does anyone have suggestions as to how to quantify canopy coverage in each photo? Many thanks! -Jeff ________________________________ ************************************ Jeffrey D. Corbin Associate Professor Department of Biological Sciences Union College Schenectady, NY 12308 (518) 388-6097 http://jeffcorbin.org ************************************ ________________________________
