Reading the TerrAvion Digest.
The TerrAvion Digest provides a quantitative analysis of what you have observed in your imagery. Digest allows growers to view statistics of single or multiple fields for a given week. For growers with large acreage, studying every field image per flight cycle is impractical. Digest allows growers to quickly identify outliers, and focus on fields that merit further attention. Growers can use digest to conduct detailed studies of a field over time, while using it to make real time decisions on a week to week basis. Digest is also a useful tool for customers conducting academic research or chemical trials on treated and untreated fields. TerrAvion Digest provides a numerical representation of the vigor in each field, allowing you to conduct statistical analyses.
Reading the Digest table
The Digest table delivers numerical statistics on the average vigor of a field, and 3 derived vigor zones (Base Mean, High Vigor Mean, Third Vigor mean).
The Mean is the general numeric average NDVI of the whole field as a single population. This value encompasses values for canopy, cover crop, and bare soil/rock.
The Base Population Mean is the mean specific to the low vigor population, useful for tracking the vigor of cover crops and soil pixel. However, if only the Base Vigor Mean is displayed, it indicates that there was not enough separation between the canopy and soil. This may be true for a high vigor field with overlapping vegetation, such as a dense potato pivot or corn field.
The High Vigor Mean is the mean specific to higher vigor populations, such as the canopy. The presence of High Vigor Means and a Base Vigor Mean indicates that we are able to detect a bimodal distribution of the relative vigor within your field.
The Third Vigor Mean is a mean specific to middle vigors. This mean is only active in rare cases where the algorithm is able to detect a distinct vigor distribution between the Base and High vigors. The presence of a Third Vigor Mean usually indicates that our system identified a cover crop as its own vigor distinct from the Base (soil) and High Vigor Means (canopy).
The Model Fit tells you how many vigor distributions the algorithm has detected. If the Model Fit number is 1, the majority vigor of the field is the same value. If the Model Fit number is 2, the field will contain both a Base Vigor Mean and a High Vigor Mean. If the Model Fit number is 3, the field contains Base, Third, and High means, which can indicate the soil, cover crop, and canopy, respectively.
The Mode is the most common NDVI values among all pixels inside a particular field.
In the images above, the histogram on the right refers to the field on the left. On the histogram, there are two distinct peaks, a lower vigor population peak and a high vigor population peak. We assume that these populations had normal, bell curve distributions.
In the image below, you can see how a bimodal (two peaks) distribution produces two distinct distributions and warrants their own means. The blue curve represents the Base Vigor Mean, gathered from the low vigor population. The green curve illustrates the High Vigor Mean, gathered only from the high vigor population. The red curve shows the overall mean for the whole field as a single population. Occasionally, there will be a third peak on the histogram between the Base and High Vigor Means, if there is a distinct distribution peak; this would be represented by the Third Vigor Mean
Each vigor mean is accompanied by a standard deviation. The standard deviation indicate the magnitude of variance in the field relative to a particular vigor mean. A low standard deviation indicates that the vigor of that field is mostly uniform. A high standard deviation indicates a large range of vigor throughout the field, which likely indicates that the field is growing unevenly.
The Base Vigor Standard Deviation, High Vigor Standard Deviation, Third Vigor Standard Deviation is the standard deviation of each mean.
Choosing Statistics for Specific Images and Dates
You can use the TerrAvion Digest to analyze statistics for a particular field over time, or for multiple fields within a week.
After selecting a field, click the Go to Statistics link in the Layer Information part of your screen. When you click on the link, you will be shown the Digest information for the field during a specific flight cycle.
At the top of the table, you can chose the date range for which you would like to see information of the selected field. Expanding the range allows you to see all the information for the selected dates, which will enable you to understand statistical changes in your field over time.
Click on the All Fields button at the top of the table to see statistics for every field in your account for a specified week. You can arrange the table by field name, date, sun angle, or any data category at the top of the page by clicking on a label.
The Digest calendar week starts on Monday and ends Sunday. Keep that in mind when selecting a Week Of date. When viewing the statistics for All Fields, be sure to set the date for the Monday of the week you want to see statistics for. Setting the date for Sunday will return results from the previous week.
You can also download the Digest table as a spreadsheet by clicking Download csv at the top of the page. The downloaded spreadsheet will provide additional histogram data, in addition to what is shown on the Digest table.