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.  We suggest comparing within the growing season, or across growing season years.  We discourage computing NDVI change during spring and fall seasons because the timing of the start and stop of these seasons can vary from one year to the next.  The shift in the timing of the start and stop of spring and fall seasons (across years) could result in mapping vegetation's phenological shift, instead of mapping real forest disturbance. Winter to winter NDVI change (within or across years) is appropriate when needing to map winter, evergreen NDVI change. 

The cloud-free quality of the 'pre-' and 'post-disturbance' time periods are easily previewed by turning-on their 'true-color' representation 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 that gets produced, can identify those areas where false NDVI change may have occurred. As a option to using multi-date compositing for the pre- and/or post-disturbance dates, see the "RESOURCES" #6 below for an app to help identify single dates of cloud-free images to use for the pre and/or post time period. 

HiForm NDVI 'absolute' change script (requires a GEE account and Google Chrome) (legend blues to -3)
https://code.earthengine.google.com/8097354c8600132b51e93599a8d2d190

HiForm NDVI 'percent' change script, strongly exaggerates NDVI change (legend blues to -1)
https://code.earthengine.google.com/6b21fa1cea97114de993a5282b12404a

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
  
   

The following workflow assumes you have an account and are viewing HiForm's data entry interface:
One popular way to quickly get a quality 1-year NDVI change product, for the 'pre' dates, is to 'max-composite' over the previous years entire growing season.  For example, enter 2021-07-15 in the first 'pre' date box, then enter 2021-09-15 in the second 'pre' date box, and for the 'post-disturbance' date, use #6 below to find one cloud free 'post' date.  In the 2-box 'post' date control windows, enter that exact single date in the first 'post' box, then add a day to the date for entry into the second 'post' date box. For example-purposes-only, if your one 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 GEE needs it referenced when denoting the use of a single date of imagery. Then in HiForm, select "Do the change analysis" to view your result.

RESOURCES

1. HiForm 'how-to'
2. HiForm Legend ( absolute )  ( percent )
3. 'Color map' file (esri .clr)
4. HiForm Logo
5. HiForm forest disturbance map products on AGOL (select the layers icon to see the table of contents)
6. Image pre-viewer to id cloud free, single dates - EO Browser (from ESA, multiple satellite platforms)
7. Image pre-viewer to id cloud free, single dates - EE Application (Sentinel-2 Surface Reflectance products)

ADVANCED SCRIPTS (no HiForm GUI)

1. Export a 10m Sentinel-2 image in natural color (script), ag-enhanced and CIR versions viewable
2. Export a 10m Sentinel-2 image in natural color and NDVI change product (script)  ('how-to' pdf)
3. Exports forest disturbance polygons directly from Earth Engine (ex: Talladega NF tornado 03/30/22)
     link to the 'how-to' pdf  ( FS )   ( non-FS )
4. Merge Landsat 8 and 9, absolute and percent NDVI change (script)

PRESENTATIONS

• See the "About" page.