Time and Tide Wait for No Man – or Imagery Collection

Smart Coastal Data Users Know Tidal Stage

About a year ago, fellow blogger, Keil Schmid, wrote about tidal coordination for lidar collection. He so eloquently spelled out why you should be aware of the tidal stage during lidar collection and some of the issues that may arise. Depending on location, coastal imagery collections can be greatly impacted by tide as well.

Is Your Feature of Interest Impacted by the Tide?

The tide level when your imagery is acquired may or may not have an influence on what you are trying to capture. If you are using satellite or aerial imagery to measure forest stands or create an inventory of backyard swimming pools, then the tide stage isn’t going to affect you. If you are creating a shoreline map, identifying oyster beds, mapping submerged aquatic vegetation, or monitoring shoal development, then the tide stage may be very important. For aerial-based imagery collections you can work with the data provider to establish flight parameters to capture your imagery when it is best (high tide, low tide, negative tide, etc.) If you happen to be using satellite imagery, you have much less control.

Low or High Tide Makes a Difference in Landsat Imagery

As an example, consider the below images. The images show Landsat imagery collected under near low (left) and high tide (right) conditions. How do I know this? I know that Landsat passes overhead in the late morning (generally around 10:00 or so), and this can be verified by looking in the accompanying metadata. I then went to the NOAA Center for Operational Oceanographic Products and Services (CO-OPS) webpage to look up tide conditions for the day the images were collected (also within the metadata).

Landsat imagery of Edisto and Charleston at low tide

Landsat imagery of Edisto ad Charleston at high tide

Date	Tide Stage	Height (relative to MLLW)	Local Time
9/3/2010 (right image)	Higher High	7.10′	9:15 a.m.
9/11/2010 (left image)	Lower Low	0.87′	8:18 a.m.

From the above images, I ran a simple classification to separate land from water to get the below images. As you can see, the extent of water is dramatically different between the two classified images. If I was interested in mapping tidal flats or coastal marsh vegetation, I would probably not want the high tide image, as much of my target of interest is covered by water.

Landsat imagery classified for water at low tide

Landsat imagery classified for water at high tide

Higher Resolution Coastal Imagery are Impacted More

A second example can be seen below using high-resolution aerial imagery collected through the NOAA Integrated Ocean and Coastal Mapping Initiative. The left image was captured under a Mean Lower Low tide stage, while the right was captured at Mean High Water. Much of the oyster beds (red circle) and mud flats (yellow circle) that are exposed under low tide are barely visible under the high tide conditions.

High resolution aerial imagery at mean lower low tide showing mud flats and an oyster bar.

High resolution imagery at high tide. Mud flats and oyster bar are obscured.

Tidal Stage Data is Critical to Accurate Analysis

If you are a user of pre-collected data and tide has an influence on your item of interest, I would strongly recommend that you try and determine the tidal condition when the imagery was collected. Ideally, the date and time the images were collected should be in the metadata. You can then use the CO-OPS historical tide data to verify. If you are planning on collecting new imagery, I would suggest using the same site to determine the predicted tides and plan around those.

There are plenty of other factors that should be considered when planning for imagery collections (sun angle, wind, water turbidity, season, etc.) but those are topics for future blogs.