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#1 MeasureGeo#2 Morse#3 Deer Awareness#4 Starship Lander#5 ImprovArticle When Satellites Go Quiet, People NoticeOn X Watch Agadez grow (be built), then handed back, to now. N...
Marshall Frith

iOS · iPhone and iPad

Sentinel-2 Clear Days — Support

Clear-sky satellite timelapses from public imagery, with optional Ships and Depth modes for learning. No account, sign-in, ads, or analytics SDK.

Get it on the App Store
Marshall Tyler Frith

Sentinel-2 Clear Days is an iPhone and iPad app published by Marshall Tyler Frith. Pick an area, and it finds the days when public Sentinel-2 satellite pictures of that place were clear, then builds a timeline you can scrub, play, and export as a short film.

This page is the App Store Support page for Clear Days. The product page is marshallfrith.com/cleardays/ and the write-up is When Did This Ground Change?.

Getting help

Questions, bug reports, or App Store review contact go through the contact form. Messages reach Marshall Frith directly. There is no in-app chat or ticket system.

Contact

Use the contact form with Clear Days in the subject. It helps to say what you were doing, the area and dates you picked, and which iPhone or iPad you use. If an export looks wrong, say which one (film, ATAK, KMZ, or a Ships or Depth file).

New in 1.6.0

  • Ships mode (optional, off by default). Learn how shipping lanes and chokepoints look from space. Pick a chokepoint such as the Dover Strait, the Strait of Gibraltar, or the Panama Canal approaches, or use any area, and Clear Days looks for ship-sized bright spots in free public Sentinel-1 radar images. It compares them with a small sample of recorded ship positions that comes with the app, or with a CSV file of positions you import. Star the ones you find interesting and play them back as a short card tour film. Detections are estimates from public data, not live tracking and not an identification.
  • Depth mode (optional, off by default). See water depths inside the area box or a polygon you draw, from public NOAA, USACE, and GMRT data, as a map layer, a 3D seafloor, and exports. The app and its depth files show depths in metres. This page gives each depth in metres and feet. For learning and planning only. Not for navigation.
  • iPad. Clear Days now runs on iPad in portrait and landscape, with a wider layout. Share sheets open from the button you tap.
  • Export for ATAK can now add the Ships and Depth layers to the data package.
  • Place search now uses Apple Maps.
  • The main Get depths and Find ships buttons stay pinned at the bottom of their panels at every text size.

How to use

  1. Pick an area: drag the box on the map, search for a place, pick a Quick start place, or tap My location.
  2. Pick a date range, then tap Scan dates to find the clear days.
  3. Tap Build timeline, then play it or step through the frames.
  4. Tap Export film, open the 3D view, or use Export for ATAK or Export KMZ to share the frame on screen as a map overlay.

Turning on Ships and Depth

Both modes are off when you first install the app. Open About in the toolbar and turn on Ships mode or Depth mode. Each adds a button to the toolbar and uses the same area box as the timeline.

  • Ships: pick a preset from Chokepoints (or keep your own area), tap Find scenes, then Find ships. Tap the circles to pick detections, or use Select by confidence. Tap the star to mark a ship, then play the Card tour.
  • Depth: use the area box or draw a polygon, then tap Get depths. Long-press the map to read a depth. Open the 3D seafloor view to see the shape of the bottom, or tap Export to share the depth files.

Reading the depth result

Not for navigation. Depth mode is for learning and planning only.

  • The summary counts soundings and gives the depth range in the grid. Dry or land points are left out of that range, and the summary says how many, for example "12 dry/land points left out."
  • Shallowest charted sounding names the shallowest measured sounding (NOAA chart or USACE channel survey) when there is one, and falls back to the shallowest grid cell when there is none. The map's grid fills in between soundings with a model, so a grid cell can read shallower than the "Shallowest charted sounding" line. Both are planning numbers only.
  • Depths are below mean lower low water (MLLW). When NOAA Tides & Currents has a station for the area, a tide line names the station, its distance, the water level now and whether it is rising or falling, and the next high or low, for example "Next high is +1.1 m at 1:14 AM EDT." (+1.1 m is about 3.6 ft.) The time always shows its time zone, and adds the date when it is not the same day as the pull. A charted depth plus this tide is the water estimate. The long-press readout adds the tide for you.
  • If an NWS coastal forecast covers the area, its line is shown under the tide.

Exports and imports

  • Export for ATAK makes a data package with the frame as a map overlay. With Ships or Depth on, you can add the shown ships, or the depth soundings and contour lines, as extra layers. The depth layer leaves out dry and land points. The depth marker icon is stored inside the package.
  • Export KMZ makes the same overlay for other map apps that open KMZ files.
  • Ships results export as GeoJSON and KMZ, and as a card tour film.
  • Depth results export as KMZ, CSV, GeoJSON, GPX, and a PDF chart sheet, all in metres and all marked "Not for navigation". When the files are ready the panel says, for example, "5 depth files written."
  • Every depth file (KMZ, CSV, GeoJSON, GPX, and the PDF chart sheet) leaves out dry and land points, like the summary. On the PDF each sounding is a dot coloured by the same depth bands as the legend ("Sounding dots:").
  • If an area has no water, the app says so and does not offer a depth export.
  • Add overlay lets you pick a KML or KMZ file from Files and draw it on the area map. The overlay stays on your device. Links to web addresses inside a file are ignored.
  • Import AIS CSV in Ships mode lets you compare detections with your own file. You choose which columns hold the vessel number, time, latitude, and longitude.

Every export is made on your device and only leaves it when you share it through the share sheet.

Common notes

  • Searching for and downloading imagery needs an internet connection. Saved areas, built timelines, overlays, and saved 3D views stay on your device.
  • You can say no to location and still search for a place or drop a box by hand.
  • A cloudy result usually means the satellite did not get a clear pass in that range. Try a longer range, or the 2017 to present preset.
  • Ships does not track ships live. A bright radar spot can be a ship, a platform, or a wave, and a match with a recorded position is a cue, not proof.
  • Depth data comes from public government and academic sources, may be old or incomplete, and must never be used to navigate a vessel.
  • If the map shows a spot shallower than the "Shallowest charted sounding" line, that is expected. The line prefers measured soundings (NOAA chart soundings and USACE channel surveys), and the map spot is a modelled grid cell between them. Neither number is for navigation.
  • Points counted as dry or land in the depth summary are left out of the map and of every depth file.
  • Deleting the app removes its saved areas, overlays, imports, and saved views.

Privacy in one line

No account, sign-in, ads, or analytics SDK, and no Clear Days server that stores your areas or films. Full policy: marshallfrith.com/cleardays/privacy/.

Requirements

iPhone or iPad with iOS 18.0 or later (iPadOS 18.0 or later on iPad). No account needed.

App Store

Price $1.99 · iPhone and iPad

Sentinel-2 Clear Days on the App Store