Understanding NDVI colormaps and selecting the one to fit your needs
Normalized Difference Vegetation Index (NDVI) is a ratio of red light to near-infrared light that our sensors detect. Photosynthetic activity and leafy green areas, in general, increase NDVI values. Our colormaps take a range of NDVI values and assign a color to it, which helps you understand numerical analysis in a visual format.
TerrAvion offers multiple colormaps that can be used to render the same numeric NDVI value visually. Some colormaps are Wide Ranged, used to make general observation over a large NDVI palette, others are Narrow Ranged, used to spot granular NDVI differences in your fields, and some focus only on specific NDVI ranges of interest.
The actual NDVI numeric values for a particular image is constant. However, different colormaps cause the same NDVI value to render in a different color on a map. The reason for this is that sometimes, a particular colormap's range is either too wide or too narrow and not showing the required detail in vigor in a particular field. It is common to change to different colormaps a few times a season.
The following examples show the same field utilizing different colormaps.
Wide Gradient Colormaps
The "Wide Range" colormap has the widest gradient of all TerrAvion NDVI colormaps.
This colormap is useful for making general observations of a field with wide-ranging NDVI values. This colormap has less variation across the whole NDVI spectrum and gives you an image that's less noisy and easier to read.
A colormap with large color gradients, such as the "Wide Range" colormap, allows you to make more general observations on fields with NDVI values distributed between a wide range on the NDVI scale.
Using our Vigor Histogram tool, 98% of our NDVI colors fall into the 3 upper color ranges of this colormap, with 77% of our field shown in a dark blue. This usually implies that the colormap is too wide for this particular field.
Narrow Gradient Colormaps
The scale above is the "Granular" colormap. The Granular colormap has one of the narrowest gradients of any TerrAvion colormap scales.
This NDVI image, with the Granular colormap, shows much more vigor detail than the previous "Wide Range" colormap. The narrower the color gradients on the color scale, the more NDVI numeric values are visually rendered, which entails more variation and detail.
Here, the same percentage of vigor is illustrated by six color values instead of three. The dark blue color range in the "Wide Range" colormap that encompassed 77% of the first image is broken down into three separate color ranges.
It is important to note that the histogram tool only shows the 9 highest NDVI values for a particular colormap, and sometimes, there are outlying values that are not shown on the graph.
TerrAvion also offers colormaps that focus on a small range of NDVI values, while muting the rest. You can find the NDVI values of your field by referring to the Vigor Histogram.
On the scale above, the NDVI values from 2.3 to 4.5 is represented by a full spectrum of colors, while all values falling outside of that range are muted. A targeted color scale provides greater detail by focusing on specific NDVI value ranges. You can see this detail in the image below.
Targeted colormaps, such as this, can be very useful in monitoring different crops at different points in the season. For instance, as corn grows throughout the season, it's NDVI values will move up the scale in the cohort. A colormap that focuses on a lower NDVI range may be best suited for earlier in the season; while a colormap that focuses on a higher NDVI range may be better suited for later in the season.
Colormaps to fit your needs
TerrAvion offers many different colormaps to help you farm profitably. The same NDVI numerical value can be rendered visually using different colormaps. Choosing a colormap that best suits your needs at any given point in the season allows you to get the most value out of TerrAvion’s NDVI imagery.