Estimate Feature Heights

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Lesson 1 of 1

Estimate Feature Heights

In this quick guide, you will:

•

Use the SAR Height Estimation tool to measure the heights of man-made objects in a Single Look Complex (SLC) image. * •

Use the SAR 3D Point Estimation tool to estimate topographic heights of individual points along the terrain. This requires two overlapping images acquired from different viewing angles. The tool exploits stereographic views to calculate precise x, y, z positions.

Although the examples in this quick guide use SLC images, you could also use power images with slant-range or ground-range geometry.

Sample Data

The exercises in this quick guide use three Umbra SLC images for demonstration. Download the ZIP file below, and extract the contents to a dedicated directory on your computer.

[SAREssentials_HeightEstimation.zip

235.6 MB

DownloadArrow down with horizontal line beneath it](assets/SAREssentials_HeightEstimation.zip)

Buenos Aires Dataset - Object Heights

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File name: Umbra_BuenosAires_slc. Also included are an associated ENVI header file (.hdr) and an ENVI SARscape metadata file (.sml). * •

Acquisition date: 25 May 2024 * •

Processing notes: The source dataset was a Sensor Independent Complex Data (SICD) file in National Transmission Imagery Format (NITF), which requires the ENVI NITF/NSIF Module to read. Since not all users have the ENVI NITF/NSIF Module, we used the "Imported data" option in the SAR Basic Data Processing tool to create an SLC image in SARscape format instead of SICD. * •

Source: Umbra Open Data Program(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source ID is 2024-05-25-13-01-21_UMBRA-04_SICD.

Hainan Island Image Pair - 3D Terrain Heights

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File name: Umbra_HainanIsland_1_slc and Umbra_HainanIsland_2_slc (including associated .hdr and .sml files) * •

Acquisition date: 25 May 2024 * •

Processing notes: We used the SAR Basic Data Processing tool to create SLC in SARscape format. * •

Source: Umbra Open Data Program(opens in a new tab), Attribution 4.0 International (CC BY 4.0)(opens in a new tab) license. The source IDs are 2023-01-23-13-46-55_UMBRA-04_SICD and 2023-01-29-01-59-33_UMBRA-05_SICD.

Estimate Object Heights

For the first exercise, you will use the SAR Height Estimation tool to measure the heights of vertical, man-made objects in an SLC image. To accurately estimate heights, the SLC image should have:

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An incidence angle of no more than 75 degrees (ideally less than 60 degrees); and * •

A height sensitivity of no more than 20 meters (ideally less than 10 meters)

A recommended first step is to run the Check Data Compatibility tool with your intended image and the Height Estimation option. The Report panel will display the incidence angle and height sensitivity value; for example:

For this quick guide, we will skip the Check Data Compatibility steps. Refer to the SAR Essentials: Check Data Compatibility quick guide for more information.

1

Start ENVI. - 2

In the Toolbox, expand the SAR Essentials folder and double-click SAR Height Estimation. The SAR Height Estimation Tool appears.

3

Click the Open menu item and select Open Image. The SAR Select Image dialog appears. - 4

Click the Browse button next to Input Image. A file selection dialog appears. - 5

Go to the location where you saved the sample data for this quick guide, and select the file Umbra_BuenosAires_slc.sml. Then click Open. - 6

Click OK in the SAR Select Image dialog. After several seconds, the SLC image is added to the Layer Manager and displayed in the Image window. - 7

Click the Zoom to Full Extent button in the Toolbar.

The image shows four tall buildings, an aircraft carrier in port, and some shipping cranes. The buildings exhibit a strong layover effect, where they appear to be leaning sideways. The image is also oriented with shadows down, so objects are not oriented in an intuitive manner.

The most accurate height estimations are only possible with vertical objects such as buildings, towers, and antennas. For this exercise, you will measure the heights of tall buildings.

8

You are going to measure the heights of buildings based on bright layover, as opposed to dark shadows. Click the Mode menu item and verify that Layover is selected. - 9

Zoom to the long building shown below. It is located near the top center of the image. - 10

Click the Start button. The cursor changes to a crosshair symbol. - 11

Click on the base of the building. A yellow circle appears at that point, and a dotted line extends along the azimuth direction. The building's layover follows this direction.

12

Follow the dotted line to the top of the building as shown below, and click a second time.

The estimated height is reported in the SAR Height Estimation Tool. In the following example, the height is 125.93 meters.

13

Click the Stop button. The yellow annotations and reported measurement are erased. - 14

Repeat these steps with other buildings and other vertical objects. - 15

When you are finished, close the SAR Height Estimation tool. Then right-click on the View item in the Layer Manager and select Remove All Layers.

Next, you will use a different tool to estimate terrain heights from a pair of overlapping Umbra images.

Background on 3D Point Estimation

For this exercise, you will use the SAR 3D Point Estimation tool to estimate topographic heights of individual points in Hainan Island, China.

The tool accepts two overlapping SLC images acquired from different viewing angles. They can come from different sensors, they can have different polarizations, and they can have different orbital directions (ascending or descending). The following table summarizes the viewing geometry requirements for the input images:

	Minimum	Maximum	Optimal Maximum
Incidence angle difference	None	120°	50.0°
Azimuth angle difference	None	120°	50.0°
Common area interesection 1	10%	250,000 km2

1 The minimum common area intersection is necessary to retrieve the same feature of the same object in both views.

Use the SAR Compatibility Check tool to test if your image pair is suitable for use with the SAR 3D Point Estimation tool. See the SAR Essentials: Check Data Compatibility quick guide for more information.

The Hainan Island image pair you will use for this exercise meets the above requirements; specifically:

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The incidence angle difference is 12.3 degrees. * •

The azimuth angle difference is 23.6 degrees. * •

The Image #1 footprint area is 2.8 km2. The Image #2 footprint area is 2.5 km2. The common area intersection is 2.2 km2 (well above 10%).

Run the SAR 3D Point Estimation Tool

1

In the Toolbox under the SAR Essentials folder, double-click SAR 3D Point Estimation. The SAR 3D Point Estimation Tool appears.

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Click the Open menu item and select Open Image Pair. A question dialog appears: "All images in the ENVI layout will be closed, with the current session saved and restored at the end of the workflow. Would you like to continue?" This is for cases where one or more images are already displayed in ENVI. The prompt allows you to save the view layout for later use. - 3

Click Yes to dismiss the question dialog. The SAR Select Image dialog appears. - 4

Click the Browse button next to Input Left Image. A file selection dialog appears. - 5

Select Umbra_HainanIsland_1_slc and click Open. - 6

Click the Browse button next to the Input Right Image field. - 7

Select Umbra_HainanIsland_2_slc and click Open.

8

Click OK. Opening both images in this tool can take a while because they are imported and multilooked before they are displayed. After several seconds, the images are added to the Layer Manager and displayed in separate views in the Image window. Because the SLC images are not georeferenced to a standard map projection, the views display different locations for each image.

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In the Layer Manager, right-click on Umbra_HainanIsland_1_slc and select Zoom to Layer Extent. - 10

Repeat this step with Umbra_HainanIsland_2_slc. The images are displayed at their full extent. The first image was acquired from an ascending orbital pass, while the second was acquired from a descending pass. Thus, the images appear flipped in the vertical direction.

Determine Terrain Heights

To accurately calculate the height and x, y coordinates of a single point, you must click on the same feature and location in both images. If resolution allows for it, you should ideally select your points at the pixel level.

1

Click the Start button in the SAR 3D Point Estimation Tool. - 2

Click on a terrain feature in the left view. A yellow crosshair symbol is added to that point. A second crosshair symbol is automatically added to the same area in the right view. - 3

Click and drag the crosshair in the second view to precisely locate the same terrain feature. - 4

Right-click in the view and select Accept Location. Height values are reported in the SAR 3D Point Estimation Tool. The first value is the height above an ellipsoid. The second value is the height above the EGM96 geoid. The third value is the height above the EGM2008 geoid.

5

Click the Stop button. The yellow crosshairs and reported measurements are erased. - 6

Repeat these steps to calculate terrain heights for other features as needed.

Video Demonstration

Click the Play Video button below to watch a video demonstration of the SAR 3D Point Estimation tool.

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