Mapping Workflow

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 ver. 2.0,  NEW: export hand drawn polygons; now includes daily Sentinel-3 (requires a GEE account)
  https://code.earthengine.google.com/0f8a8e358d6f3ce7b5fef3893403a88b
    see Resources section #1 below for export formats and capabilities

HiForm Timber Harvest BMP Tool application (HiForm-BMP) (does not require a GEE account)
  County-level mapping, publicly available for the (13) southeastern states
    for more information, visit - https://hiform.org/projects/timber-harvesting
 

Sign up for a Google Earth Engine account:

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 #5' 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 for version 2.0 (FS) (non-FS):

    Script can export: (1) 'all-lands' NDVI change image, format: a single-band continuous greyscale, signed 16-bit raster, requires a color map to replicate the HiForm color ramp of NDVI change (requires a forest mask be applied to isolate forest-only change); this product can be used to threshold and isolate a specific range of NDVI change values; selecting the button "Export image to Drive" automatically executes and stages the task of exporting the 'all-lands' NDVI change raster.
     

  2. HiForm Legend 
     
  3. 'Color map' file (esri .clr) for the ndvi change download
     
  4. HiForm Logo
     
  5. Use the EO Browser to find dates of cloud free imagery