NDVI

What is NDVI?

Normalized Difference Vegetation Index (NDVI) is a ratio of the red light to near-infrared light that our sensor sees.  Plants are uniquely efficient at reflecting light in a way that increases the value of this ratio.  Higher NDVI values mean more chlorophyll is detected by the sensor.  Both leaf area and photosynthetic activity increase NDVI values.  Normally, these two values are conflated into a measure of “vigor,” but basically it is a measure of chlorophyll density from whatever cause.

Technical Description of TerrAvion NDVI Values

TerrAvion’s NDVI is translated to an 8-bit scale which has pixel values from 0-255, however our NDVI values do not span the whole range.  0 corresponds to the start of the useful range of NDVI values, not zero in a reflectance calculation and 255 is off the scale.   The start of vigor, i.e. the sensor is seeing living growing chlorophyll is about 50 on our scale.  Non-living materials will have values below 50 and plants that are growing will measure more based on how vigorous they appear to the sensor.  Each NDVI value is the same vigor consistently throughout the season, this allows comparisons about photosynthetic potential between locations and through time.

Which NDVI Color Scale Should I Use?

Wine Grapes:

NDVI values for wine grapes range from 0.25 at the lowest (stressed) up to 0.5 (vigorous). Depending on the varietal and the trellis style, your grape canopy will likely occupy a sub-range between those two values.

Default
Gradients in Default are wider so the vigor information will appear more tiered. The blues between 0.4 and 0.5 are not easy to distinguish so this color map is better suited to a crop that is in a lower subrange, such as 0.25-0.4

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AVHRR
Originally developed by NASA, gradients in AVHRR are approximately 0.03 wide up until 0.4. Like default, this color map is suited to a lower-vigor crop as any vigor differences from 0.4 above will not be distinguishable. 

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JG 0.5 Step Rainbow
Gradients in this color map are approximately 0.05 wide which means they are slightly wider than Default and AVHRR however distinct gradients continue up to 0.8 so this color map may better suit a high-vigor varietal. Many users remark that the color scheme is familiar to them.

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Granular
Gradients in this color map are 0.025, the narrowest of any color scale. This color map enables maximum variability. Values below 0.2 are muted with brown and gray to represent non-plant data. This color map is suitable for any varietal.

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GM Quad
With no sharply divided gradients, this color map has a multi-colored fading gradient from 0.2 on. Values less than 0.2 are muted with gray. Changes in vigor less than 0.1 will appear very subtle, resulting in a textured look. This color map is best suited for flipping through multiple weeks at a time to observe a more gradual trend of change.

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Tricolor
Gradients in this color map are at a narrow 0.025 to maximize the presentation of variability in crop performance. High-vigor crops benefit from this color map as the main area of interest is between 0.3 and 0.6.

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Because different growing practices create different amounts of vigor, we offer several different color scales to view your data.  You can see instructions on how to change between the scales here.

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The table above compares the different scales so that you can see how the colors on each scale relate to one another and the underlying values that they represent.

One final word on NDVI scales: Whatever scale or scales you use, go back and look at the natural color and color infrared images.  All the information that the NDVI has is contained in those two images, plus a wealth of other information that a grower—an expert at looking at plants—can see that the computer cannot.

How are NDVI Values Distributed?

You can see the histogram of NDVI values from the UC Davis V&E Vineyard below and the image that the AVHRR scale creates, as well as a natural color image for reference.

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I want to manipulate my data in some other format, how do I do that?

You can roll your own scale and categorization. We provide the NDVI’s as downloadable palletized .tif files that can be downloaded from the interface.  These files are geo-registered in WGS 84 Web Mercator Auxiliary Sphere (EPSG: 3857) and they contain NDVI values from 0-255 in Band 1, with corresponding mapping to the NDVI function of that pixel.   These files can be downloaded into GIS programs such as ArcGIS or QGIS.  There you can stretch the NDVI histogram, classify, or run pixel math to unlock more information.  If you are going to do this at scale, contact us about an API for the program you are using.

Call us if you have questions about any of this.

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Tech Specs?

Our imagery has been calibrated in relative reflection units.  This allows you to convert our GeoTIFF NDVI values into the standard reflection unit NDVI using the following formula:

NDVI = 0.0046 x – 0.046
where x is the NDVI GeoTIFF pixel value.

You can also use reference the charts below.