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).
|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.
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.
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.
Hi John, can you please help me? I am looking to get low tide images of Darwin Harbour in Australia. All of google earths pictures the tide is way higher than I need. Can you please put me in the right direction, I am sorry to bother you with this but thought its worth a shot.
I would suggest trying to find other imagery sources that may be available. Imagery is commonly collected by different levels of government agencies. I did a quick web search on “northern territory Australia gis”, and the first link took me to http://www.nt.gov.au/ntlis/. There appears to be many links with potential imagery sources for you. A second piece of info you would need is tidal information, which can be found here http://www.bom.gov.au/oceanography/tides/MAPS/nt.shtml. From here, hopefully you can find some imagery, and it’s associated metadata (for time/date of collection) and match that to the tide data.
Hope this helps.
I like your explanation about ow and high tides but would you please help me to get the tide satellite imagery portal where i can download imagery with respect to my area of interest, because i want to study a certain part of west Indian ocean and see how do tides vary in a day, month and yearly
I’m not certain what you’re referring to as a “tide satellite imagery portal”. If you want to study the tide cycles at a location, you generally need a tide station, not satellite imagery. What you’ll find may depend a lot on which country in that area would have tide stations that work for you.
To add to the previous comment. “If” you are looking for imagery collected at specific tidal stages, there would be a couple steps. As mentioned above, you will need to determine when your specific tide of interest occurs. Here in the US, we can refer to NOAA tidal stations. You could then compare the time of collection for your various satellite imagery. In the Blog, the metadata with the Landsat imagery lists the time of collection, which I compared against the tidal data.
ok, thanks but how did you get these images for low tide and high tide shown above, where did you download
It mentions that in the post. The local time of overpass was known for landsat (which is sun synchronous, so that stays the same) and then he looked to find dates where that time would match the desired tide stage.
I used GLOVIS (https://glovis.usgs.gov/) for the Landsat data. I’m not familiar with data repositories for the western Indian Ocean, but I imagine some exist for remotely sensed data.
Thanks a lot, i like your patience and help, so let me try digging