Detecting and measuring coastal change
Coastal change is a global phenomenon that is attributed to tides, powerful sea currents and overall climate change. It has the potential to threaten communities and local economies of coastal towns and cities. Accurate detection and measurement of coastal change can provide valuable data for scientific studies of coastal evolution, as well as guide flooding disaster preparedness and mitigation.
We developed an end-to-end GBDX workflow for coastal change detection and measurement at the native resolution of our 8-band multispectral imagery. All you need to run the workflow is a GBDX account and Python. The workflow can measure coastal changes at the meter level and at unprecedented scale.
The example we cover here focuses on Cape Cod, an area which is well known for extreme changes in the coastal landscape. The workflow takes two roughly collocated images of Cape Cod, captured in 2010 and 2016 by WorldView-2 and WorldView-3, respectively, and computes coastal change on the entire images, roughly an area of 1500km2 in less than 30 minutes.
- Measuring change
- Running the workflow
We’ve picked two images of Cape Cod (MA); the details are shown in Table 1. Click on each catalog id to view the imagery.
|field||image 1||image 2|
|off nadir angle||12 deg||16 deg|
Table 1. Image attributes.
Note that this workflow requires imagery from either WV2 or WV3; it is only these two satellites that provide high-resolution (< 2m) 8-band multispectral imagery necessary for selected tasks in this exercise. The image acquisition dates differ by 6 years and 2 months. They were both acquired from similar angles during spring/early summer, and contain no clouds. Coming from different sensors they are of different spatial resolution and they overlap for most of their extent. Figure 1 shows the image bounding boxes projected on Google Earth; the red frame marks the intersection.
Figure 1. Image bounding boxes and intersection marked in red.
We ordered the two images, ran them through the Advanced Image Preprocessor and stored them in S3 - see Figure 2. What preprocessing involves in this case is ortho-rectification, projection to UTM coordinates, and atmospheric compensation.
Figure 2. Image preprocessing.
Our workflow is shown in Figure 3. Green boxes correspond to GBDX tasks and blue boxes to images or information layers. image1 and image2 correspond to the post- and pre- image. However, keep in mind that the workflow is symmetric with respect to the chronological order and image1 and image2 can be interchanged.
Figure 3. The coastline change detection workflow.
The workflow starts with the Image Pair Alignment (IPA) task which takes as input two AOP-ready images, computes their geographic intersection and returns two new images, each containing the subset of the original that lies within the intersection. After successful conclusion of the IPA task a water mask (WM) is computed from each IPA-ready image. A water mask is a binary image in which white or ‘foreground’ pixels correspond to automatically detected water in the original, and black or ‘background’ pixels to everything else. The two images are also used to compute a single exclusion mask (EM). The EM is a the sum of all image regions that might generate a change with respect to the target class (water in this case) that is irrelevant to the workflow, such as cloud cover and no-data zones. The point of the EM is to remove these regions from the final change detection map.
The three masks are fed to three different Binary Change Detection (BCD) tasks, GBCD, LBCD and TBCD. GBCD and LBCD produce binary images that show the water gain and loss, respectively. TBCD delivers a tristate image that shows water gain in green, water loss in read and no change regions in white (and black corresponding to regions masked out by the EM).
In many cases, we need to produce semantic layers, e.g., contours of coastline change, or statistics such as the change mean or variance. For this purpose, we have created two additional tasks (see Figure 4) which use the water gain and loss maps to produce change detection distance maps. See this section for details.
Figure 4. The distance transform tasks of the change maps.
The tasks used in the workflow are listed in Table 2.
|Name||Acronym||GBDX Task Registry Name||Description|
|Image Pair Alignment||IPA||protogenV2CD_READY||Image preparation for change detection|
|Water Mask||WM||protogenV2RAW||Water layer extraction|
|Exclusion Mask||EM||protogenV2CD_LULC||Computes an exclusion mask based on selected land use land cover classes|
|Binary Change Detection; gain||GBCD||protogenV2CD_BIN_GAIN||Computes the change map between two binary input layers and returns the gain with respect to the most recent one|
|Binary Change Detection; loss||LBCD||protogenV2CD_BIN_LOSS||Computes the change map between two binary input layers and returns the loss with respect to the most recent one|
|Binary Change Detection; tristate||TBCD||protogenV2CD_BIN_TRI||Computes the change map between two binary input layers and returns the tristate output: gain in green, loss in red, white for no change, black for irrelevant|
|Gain map Discrete Distance Transform||GDDT||protogenV2CD_GDDT||Computes the distance map of the gain map|
|Loss map Discrete Distance Transform||LDDT||protogenV2CD_LDDT||Computes the distance map of the loss map|
Table 2. Workflow tasks.
If you want to find out more about each individual task, read on.
Image pair alignment
The IPA processor takes as input two AOP-ready images. It verifies that they are in the same coordinate system, it computes their bounding boxes (black dotted lines in Figure 5) and checks if these overlap geographically. If they do not overlap an error is returned and the workflow terminates. Otherwise, the IPA processor computes their geographic intersection that is a bounding box in itself (orange dotted line in Figure 5) and it stores it as a vector.
Figure 5. IPA processor, step 1/4: Computation of the intersection.
The IPA processor uses the intersection vector to extract the AOI (i.e., the contents) of each image that lies within it, and save it in a new image. If the two AOIs differ in spatial resolution, the high resolution AOI is downsampled using bilinear interpolation to the lower resolution. These steps are shown in Figure 6.
Figure 6. IPA processor, steps 2-4: AOI extraction from each image using the intersection bounding box, and resampling, if necessary, for resolution normalization.
A WM of an image is a binary image where white (foreground) pixels correspond to water bodies and black (background) pixels to everything else. The GBDX task which generates WMs is protogenV2RAW. protogenV2RAW is completely unsupervised; it uses the image spectral information and certain morphological operations in order to locate water bodies.
Screenshots of the pre and post WMs are shown in Figure 7.
Figure 7. Pre and post WMs. White is water and black is land.
The exclusion mask (EM) is the sum of all image regions from both IPA-ready images, that may generate a change with respect to the foreground class (water) that is irrelevant to the objective. For example, consider a cloud-covered region in one of the two images that is found within a water body. If the second image does not contain the same cloud in the same area, a change will be registered. This change is irrelevant to coastal change and needs to be marked as such.
The EM is a binary image computed from the Land Use Land Cover (LULC) layers of the input images. In this workflow, the EM consists of the sum (logical OR) of all cloud and no-data zone pixels in both input images. The EM is generated using the protogenV2CD_LULC task which we created for this purpose.
Figure 8. The EM task takes as input two IPA-ready images, computes the respective LULC layers internally, identifies the foreground class and constructs the output image using the preselected class blending operator; in this case, logical OR.
Figure 8 shows the intermediate and output layers of the exclusion mask creation task. Note that on the output layer the two foreground regions at the center of the image are LULC layer errors (classes misinterpreted as clouds). The frame around the image is made of no-data zones accumulated from both input images.
Binary change detection
Change between the WMs is detected on a pixel by pixel basis. If the pixels have different values the result is 1, else it is 0 (an XOR operation). The workflow deploys three different BCD tasks; protogenV2CD_BIN_GAIN for water gain detection, protogenV2CD_BIN_LOSS for water loss detection and protogenV2CD_BIN_TRI for water gain/loss/no-change detection. Screenshots of the three outputs are shown in Figure 9. In the tristate image, green, red and white designate water gain, loss, and no-change respectively, while black highlights the regions marked by the EM as irrelevant to water change.
Figure 9. BCD inputs and outputs.
The water gain and loss change maps permit the visualization of change but do not contain information about its magnitude. We implemented two GBDX tasks, protogenV2CD_GDDT and protogenV2CD_LDDT, that apply a Discrete Distance Transform (DDT) on the gain and loss change maps. The DDT encodes in the intensity of each pixel in regions that have undergone change, its closest distance from the coastline in the pre image. In other words, the pixel intensity reflects the degree of change.
Measuring water loss
The next figure shows an area of extensive water retreat.
Figure 10. Pre (top) and post (bottom); water retreat.
Figure 11, left, shows the DDT of the loss map. White pixels indicate water which is present in the pre and post. Grayscale pixels indicate the region of water loss; the brighter the pixel, the larger the distance from the pre-coastline. Black pixels correspond to everything else. For this visualization, we converted the 32-bit output of LDDT to 8-bit which results in intensity saturation. As a result, a part of the outer contour of the DDT appears as white. We experimented with pseudocolor which resulted in the visual shown on the right and shows the true extent of the water retreat.
The maximum pixel intensity is 487; since the resolution is about 2m/pixel, an approximate calculation reveals that the maximum water retreat in this AOI is 974m! Note that the DDT is sensitive to pixel isotropy. Consequently, in order for these calculations to be accurate, input images given in lat,long should be converted to UTM.
Figure 11. The DDT of the loss map (top) and a visual impression in pseudocolor (bottom).
Measuring water gain
Figure 12 shows an AOI where the landscape has changed dramatically between pre and post. Water has appeared in place of land and vice versa.
Figure 12. Pre (top) and post (bottom); water gain and loss.
Figure 13, left, shows the DDT of the gain map. The two grayscale ribbons correspond to areas where water has appeared in place of land; the pixel intensity captures the degree to which the water has advanced to reach that very point. A visual impression in pseudocolor is shown on the right. The dark green line indicates that the maximum expansion has occurred in the center of the ribbon; water moved in from both directions to cover that stretch of land! In this case, the maximum pixel intensity is 188 which implies that the extent of the water gain in this AOI is 376m.
Figure 13. The DDT of the gain map (top) and a visual impression in pseudocolor (bottom).
Running the workflow
The entire workflow is found here. If you have access to GBDX, you can run the Python code therein and save the results in S3.
We had one of our team members who is a Cape Cod local have a look at the results.
Figure 14 is a screenshot of the Chatham area. The red and green areas are most likely due to tidal effects and shifting sand bars.
Figure 14. Chatham area.
Figure 15 shows the area around the former Marconi transatlantic wireless station! Red corresponds to tidal marsh which our water mask picks up as land.
Figure 15. Marconi transatlantic wireless station area.
Figure 16 shows the area around Rock harbor. Red corresponds to tidal flats which are mostly covered by shallow water. The arrow on the right points to Nauset Inlet, an area known for tidal changes and shifting sand bars. You can also barely see the remains of USS Longstreet underwater!
Figure 16. Rock harbor and Nauset Inlet area.
Finally, Figure 17 examines some of the changes observed within the peninsula. Red corresponds to seasonal water retreat while green are possible look angle artifacts.
In summary, red in the bays and on the western shoreline can be attributed to normal tidal activity. Green and red on the beaches facing the Atlantic Ocean can be mostly attributed to sand bar changes and erosion.
We ran the workflow for Cape Cod; you can easily imagine running the exact same workflow for the East Coast of the United States. What you need are the pairs of pre and post imagery; you can use our catalog to find these. You can also experiment with this workflow to detect continental water changes, along rivers and lakes.
Attempts to estimate coastal change have been made by other parties, including USGS. The key advantages of the proposed workflow are precision and scaleability; it runs on high-resolution multi-spectral imagery and it runs on GBDX. We are currently working on a more thorough quality assessment of our results.