The HiForm forest change script uses Google Earth Engine (GEE) to map changes in greenness, specifically, changes in the Normalized Difference Vegetation Index (NDVI). HiForm's efficient system of mapping NDVI change allows users to quickly explore changes in vegetation between any two periods of time using Landsat 5, Landsat 8 and Sentinel 2 satellite platforms. As snow, clouds, shadows and water have low NDVI values, HiForm uses maximum-value compositing of NDVI values to isolate and select the greenest vegetation values (image pixels) available over the specified period of time. Selected periods of time (used to obtain the greenest pixel values available) can be as short as only a few weeks within a single season, or over an entire season, or across a year, or multiple years. We suggest not performing NDVI change analysis comparing spring or fall from one year to another because the phenological timing of seasons across years often vary. This will result in vegetations phenological-shift being mapped which can mask real forest disturbance.
Clouds and cloud shadows create false NDVI change that gets displayed in the final change map. Our newest script allows you to quickly obtain cloud-free-as-possible 'pre' and 'post' datasets that are used to calculate NDVI change. The initial quality of these input datasets are easily previewed for the presence of clouds by displaying their 'true-color' image representations that are automatically created by the HiForm script. It's through this inspection that an offending cloudy date be identified via querying a 'date raster' layer. Once that cloudy date of imagery is identified, the calendar-based controls can be adjusted to refine the date range used to avoid that cloudy date from being selected and used to calculate NDVI change. Use this workflow (of image preview and refining the date selected) as an iterative process to obtain the most cloud-free range of dates when defining multi-date image composites for the 'pre' and 'post' input datasets. This workflow will produce the most accurate NDVI change map possible.
HiForm NDVI change script (requires a GEE account and Google Chrome)
Signing up for a Google Earth Engine account:
- non-Forest Service users (exports save to G-Drive)
- sign up at https://earthengine.google.com
- Google Chrome download
- Forest Service-only users (exports save to FS Google Cloud Storage)
- Account request and setup
- Saving to Google Cloud Storage
- Install Google Chrome using the FS Software Center
- HiForm 'how-to'
- HiForm Legend (.png)
- 'Color map' file (esri .clr)
- HiForm Logo
- HiForm on ArcGIS Online
- Image pre-viewer to id cloud free, single dates - EO Browser (from ESA, multiple satellite platforms)
- Image pre-viewer to id cloud free, single dates - EE Application (Sentinel-2 Surface Reflectance products)
- Non-HiForm script producing both absolute and percent NDVI change (no user interface) -