Earth Observer

Polar Explorer: Sea Level App© Instructions

Opening the App

Upon opening the app you will see a screen filled with the Basemap. The map will be at first in shades of gray and turn to color once all the required content is loaded. A visual Tutorial opens when touching the New User icon that disappears as soon as the map is manipulated. Exit the Tutorial by touching the Back button on its toolbar.

Using the App

The Basemap displays sun-illuminated relief of land and ocean floor created at Columbia University. You pan the map by dragging a finger across the tablet surface, and you zoom in by separating two fingers in contact with the glass. A thin green border around the map indicates that you can continue to zoom in. A red border warns that you have reached the maximum detail for a particular region. You zoom out by closing two fingers in contact with the glass. The map is continuously refreshed by bringing in new tiles via either a WiFi connection or 3G on the iPhone or 3G-enabled iPad. The application will not refresh beyond the inital global view without a working internet connection. When zooming or panning, the momentary yellow lines are borders of individual tiles that are being requested from servers. These yellow lines disappear after the required tiles have been fetched successfully. The content for the viewport is obtained from the internet and does not require permanent storage on your device. A limited number of recently-fetched tiles are cached on a first-in and last-out basis to improve performance and retain content in the regions recently viewed. The scale bar in the upper right of the mapview updates according to the zoom level and its geographic location.

Re-Opening the App

Upon re-opening of the app, the view appears with the Basemap at the zoom level and location of the last session. This temporary "bookmarking" allows you to continue where you left off prior to answering your phone or using another app.

Finding Content

When touched the icon in the lower right corner of the screen opens a Menu Panel over the mapview with a scrollable list of available content to overlay on the Basemap. The Done button in the lower right corner of the Menu Panel returns to the map view. The Camera button in the bottom right center of the Menu Panel creates a photo of the current map view and stores the photo as a jpeg file in your Pictures folder. The source of the layer content is credited in the lower left corner of the map view. The Gear-Shaped button in the bottom left center of the Menu Panel opens a Preferences Panel with on/off switches that allow you to enable or disable the appearance of legends, scale bar, place names and dim the Basemap in order to highlight regions where the elevations and depths are available in high-resolution. The Done button in the lower right corner of the Preferences Panel returns to the Menu Panel.

Choosing Layers

You select a layer (topic) to overlay on the Basemap by touching its title. If the "i" icon to the left of the title is touched instead, this action opens a brower with a description of the content for that layer along with data sources, credits, citations and links to other web sites for further information. Touching the Done button in the web browser returns to the Menu Panel. When a title with an arrow after it is touched, a child Menu Panel appears with a scrollable list of further layer choices related to the parent topic. The Back button returns to the parent Menu Panel. The Done button returns to the mapview with the selected layer displayed. Note that the Basemap is always the default layer unless another choice is made. Following the selection of a layer and the touching of the Done button, the selected layer will appear, but in some cases it may be necessary to pan across the screen or zoom out to the global scale in order to locate its content.

Changing the Opacity

Upon the selection of any layer except the Basemap, a slider bar appears in the bottom left portion of the map view in portrait mode or on the right side in landscape mode (on the iPhone and iPod touch). Touching and then moving the slider button to the left in portrait mode or down in landscape mode causes the overlay layer to become more transparent. Only one layer at a time can be overlaid on the Basemap.

Finding Further Information About a Layer

After choosing a layer and viewing its image in the map window, you will notice the "i" icon in the lower left corner of the map window. Touching this icon opens a brower with a description of the content for that layer along with data sources, credits, citations and links to other web sites for further information. This action is identical to touching the same icon in the Menu Panel. However, the action in the map window allows you to view the information without going back to the Menu Panel and finding the choice currently being viewed.

Displaying Values

For many, but not all layers you can tap on the displayed overlay layer and the value or description of the layer at the selected spot is shown and marked with the apex of a triangle.

Legend Appearance

Because of the limited size of the mapview when using the iPhone and iPod touch, legends only appear in the landscape mode on these devices.

Scale Bar Accuracy

As a consequence of the distorting nature of the three flat map projections, the scale bar is an accurate representation of distance only for the region directy under the scalebar.

Bandwidth Limitation

When using the iPhone with the 3G connection, the app will fetch tiles at its normal rate upto a threshold of 2.5 megabytes and then throttle down so as not to fetch more than a total of 5 megabytes of content over a five minute span. If you are not constantly panning and zooming, you will probably not notice this limitation. There is no bandwidth limitation when using the WiFi connection to the internet.

Datums and Projections

All content is transformed into and displayed in the WGS 84 coordinate frame for the Earth. WGS 84 uses the 1996 Earth Gravitational Model (EGM96) geoid, revised in 2004. The corresponding EPSG Projection codes are 3857 for the spherical Mercator Projection, 32661 for the North Polar Stereographic Projection and 3031 for the South Polar Stereographic Projection.

Offered by the Polar Explorer team at the Lamont-Doherty Earth Observatory of Columbia University.