Using the Analyze Area Tool for Ecological Monitoring
This article will cover:
- Using Analyze Area
- Area in Range
- Year Over Year comparisons
- Category breakdown charts
- Saving Analyze Area charts in notes
- Exporting Analyze Area values
Analyze Area allows you to easily visualize seasonal patterns and year-over-year changes in ecological conditions. Drill down to specific areas of interest within a property and see how conditions have changed over the last several months or years. This feature can be used in a variety of applications, such as determining the timing of a timber harvest by reviewing seasonal cycles in vegetation vigor, or visualizing fluctuations in water and surface moisture presence in wetland areas.
We derive these data from Sentinel-2 data, an ensemble of two satellites – the first launched in late 2015 and the second in 2017. The 10-meter resolution Sentinel-2 data are captured almost every week, making them ideal for tracking changes over time. You can explore details on vegetation, surface water, surface moisture, and snow for targeted areas of interest. When using Analyze Area on the S2 Vegetation layer, sometimes shadows in areas with hills and mountains can read as higher NDVI in the winter given the lower sun angle, so always cross-reference with truecolor imagery when in doubt. Find more information on these indices in the Index Layers article.
Using Analyze Area
Launching, choosing layer, selecting chart type
From the map view, you'll see the Analyze Area button on the left pane. You can either draw an area of interest on the map, select the property boundary, or choose an overlay polygon.
By default, you'll be in draw mode where you can outline your targeted area of interest by clicking to drop each point and double-clicking to close the polygon, or choose to select the whole property within the dialog box. The polygon will be locked in place by default, and you can double click to edit the shape, such as dragging it to a new area on the map or moving vertices.
If you choose "Select property" the boundary of the site will be outlined automatically. And if you "Select overlay," you'll be able to click on an overlay polygon that's visible on the map to make that the boundary for analysis.
Once your polygon is set, we'll crunch the numbers to assess how conditions have changed over time. You can choose from any index or categorical data layers and select the time period you would like to visualize in the graph. You can also make a note to associate with this particular part of your property for easy reporting. We filter out outlier data points that are more than 15% different from a rolling median. You can choose between Average Value (default) and Area in Range as chart types. Average Value will graph the average pixel value in the polygon for the selected layer.
If you'd like to view a larger chart, click the Maximize button to expand the chart area.
Exploring chart data
Once you have a graph loaded, you can use your mouse wheel or trackpad to zoom in and out on specific months and pan horizontally by clicking and dragging the chart body. To reset the visible time range, click on the refresh icon to the left of the x-axis.
To change the Y-Axis to fit the data, you can click the "Fit to Y-Axis" icon on the right side of the chart. This is handy for year-over-year charts or if you're trying to see subtle variation in trends for index layers.
Fit to Y-Axis also works for categorical charts so you can choose specific categories and zoom in further.
To visualize the data behind a point on the graph, select a point on the Analyze Area chart to view the index layer image of the corresponding date. To enter Compare Mode, select one point on the chart then hold shift and select a second point. Hover over a point on the chart to display the corresponding date and the index value at that point in time.
Area in Range
Area in Range, shown in the example below, is a chart type that allows you to define a metric range of interest and graphs the percent of the selected area within that range. You can then click the map button to the right of the slider bar to visualize the pixels in range on your property. This is particularly useful for visualizing the areas of a property with the highest vegetation vigor, or the areas of a property that experienced the highest severity of fire.
In the example above, we've narrowed the visualized range of the Burn Index layer to only the highest values in order to understand which areas of the property were most severely burned. I can see these areas on the map and also hover over data points to see the acreage and percent of the property that is within this range of values. On 6/29/2022, 260 acres or 33% of this property was within the highest burn index values.
Year Over Year comparisons
The Analyze Area year-over-year chart allows you to compare trends from multiple years on the same annualized axis. You can look at the entire calendar year or compare particular months of interest by zooming in on the graph. Use this chart to look at trends between years, find outliers, and identify areas for closer analysis. You’ll see each year of data represented by a separate line on the chart. If you prefer to hone in on specific years, you can toggle individual years on and off by clicking each year at the bottom of the chart.
Category breakdown charts
For category layers, including NLCD landcover from USGS and the European Space Agency's WorldCover data, we now display a bar chart breakdown. This can be a great way to compare land cover change over multiple years.
The chart shows % of the selected polygon by default, and you can see the area associated with each category by hovering your mouse over each part of the chart. If the data layer you've selected has multiple time steps, you can select and de-select those in the date dropdown. Currently, this type of chart cannot be saved in notes.
Saving Analyze Area charts in notes
You can also save the chart from Analyze Area in a Note. From the Analyze Area tool, simply add a description of what you're seeing below the graph and click "Create" to save this graph as a note, and be sure to check the box for “Include Analyze Area graph”. This will ensure that the chart will be included in the reports you generate about this property. You can also zoom in on the graph from the Notes pane or in the Report as needed.
Note that you cannot add a note to an Analyze Area chart created for a multi-polygon. In this scenario, the notes section will be greyed. You will only run into this issue when you use the “select entire property” option to launch Analyze Area on a multi-polygon property.
If you made a polygon note, you can open Analyze Area directly from the note by clicking the three dots on the top right. This will allow you to dig into trends in the data for that note's geometry.
Exporting Analyze Area values
To export the values from an Analyze Area chart, click on the three dots icon in the upper-right corner, and select the “Export graph CSV” option. You can also select “Export shapefile” to download the polygon used for the Analyze Area analysis.
The CSV includes three columns: date, index value, and area. If the “Average Value” option is selected, the acres column will refer to the total analysis polygon area (e.g., the entire property boundary). If the “Area in Range'' or “Area by Category” options are selected, the acres column will refer to the acreage that falls within that range at each given date.