The HiForm forest change tool uses Google Earth Engine (GEE) to map changes in greenness, specifically, changes in the Normalized Difference Vegetation Index (NDVI). HiForm allows users to quickly explore changes in vegetation condition between two time periods, using Landsat or Sentinel 2 satellite imagery. HiForm can use maximum-value compositing across multiple dates to reduce the negative effects of clouds and cloud shadows made apparent in the NDVI change product that gets produced. The two periods used to calculate NDVI change can be as short as only a few weeks, or over an entire season, or across a year, or multiple years.   

The cloud-free quality of the 'pre-' and 'post-disturbance' time periods are easily assessed by viewing their 'true-color' representation selectable in the data layers list.  Use this workflow (for previewing the dates selected) as an iterative process to visually obtain the most cloud-free inputs possible.  Sometimes clouds cannot be avoided, so viewing the pre/post 'natural color' products in conjunction with the NDVI change product can identify those areas where false NDVI change may have occurred as a result of clouds, shadows. 

HiForm Forest NDVI Change script (requires a GEE account)
  https://code.earthengine.google.com/5f405ee50389e3eaf2843f397f94e53e


Sign up for a free Google Earth Engine account:

• non-Forest Service users (exports save to Google-Drive)
  - sign up at https://earthengine.google.com
  - Google Chrome download
   
• Forest Service-only users (exports save to FS Google Cloud Storage)
  - Sharepoint; Request form
  - Saving to Google Cloud Storage
  - Install Google Chrome using the FS Software Center
  
   

EXAMPLE WORKFLOW (using the HiForm script)

A popular way to quickly get a quality 1-year NDVI change product, is to use the previous year's full growing season as the 'pre-disturbance' baseline.  For example, enter 2021-07-15 in the first 'pre' date box, then enter 2021-09-15 in the second 'pre' date box.  For the 'post-disturbance' date(s), use 'RESOURCES #7' below to find the most cloud free image date(s).  In the 'post' date windows, when denoting a single date, start with entering the exact date of the cloud free image in the first 'post' box, then add a day to the date for entry into the second 'post' date box. For example, if your single cloud free 'post' date is 08-07-2022, enter 2022-08-07 in the first 'post' date box, then enter 2022-08-08 in the second 'post' date box (that's the way EE needs it, when denoting the use of a single date of imagery). After that, select "Do the change analysis" to view your results.  Alternatively, for the post-date selection, you can always use a range of dates and allow the software to choose the most cloud free pixels over that date range; that process results in creating a single 'post-disturbance' image that will be used to calculate NDVI change.  Remember to toggle the 'post natural color image' (under 'Layers'), along with the NDVI change image, to see if clouds and cloud shadows are responsible for causing 'false' NDVI change. 


RESOURCES

1. Users Guide (FS) (non-FS)
2. HiForm Legend (absolute)  (percent)
3. 'Color map' file (esri .clr) for the ndvi change download
4. HiForm Logo
5. HiForm on AGOL (select the layers icon to see the full table of contents)
6. Use the EO Browser to find dates of cloud free imagery
    

ADVANCED SCRIPTS

1.   Creates and exports detailed forest disturbance polygons, view the 'how-to' pdf for the script's link
     (the example provided is from Forest Tent Caterpillar, on the Pearl River, LA, 04/17/2023)
2.   The "HiForm Timber Harvest BMP Tool" (script) ('how-to' pdf)
3.   Ask about the "HiForm National Forest Tool" (contacts)
    

PRESENTATIONS

• See the "About" page.