Google Earth Tutorial: Overlays
Geobrowser
The following are
notes on the GTI presentations by John Bailey & Katie Kennedy about how to
create Overlays in Google Earth. If you have questions please add a reply to
this post and we will respond as soon as we can (if someone else doesn’t answer
first!)
(1) What is an Overlay?
There are three types of “Overlay” that can be created in Google Earth. They
each represent a different way an image can be imported into Google Earth using
KML:
(i) Ground Overlays
These images are located horizontally on or above the landscape. If located at
ground level they mantle the topography (see below for instructions on how to
create).
(ii) Photo Overlays
These images are located vertically in the GE landscape as “billboards”. To
view the full resolution of the image the user must fly-into the image, at
which point navigation in the 3D browser is restriction to only that image.
Photo Overlays can be created directly in GE:
http://www.youtube.com/watch?feature=player_detailpage&v=D9LcwbwQgzk#t=40s
(iii) Screen Overlays
These images are located in the 3D browser but remain fixed and independent of
navigation in the 3D browser. These overlays are not interactive, but
transparency can be altered.
Screen Overlays cannot be created in GE; the code must be authored directly in
a text editor.
KML Examples
Each type of Overlay using images showing Mt. Etna volcano on Sicily, Italy:
Ground, Screen and Photo.
(2) Creating Ground
Overlays
There are a number of ways to add KML objects in Google Earth. The three simplest methods to create a Ground Overlay are:
(i) Use
the menus (Add -> Image Overlay)
(ii)
Click on the icon (two rectangles) in the toolbar
(iii) Right-click
on the folder in the Sidebar and select Add -> Image Overlay
Green guidelines will appear on the globe marking four corners, four edges, and a center point. Along the left-hand edge there will be an unfilled green diamond. These green guides indicate where the overlay will be located.
A dialog box will also appear, within which there is an empty white "link" box. Type or paste into this white box a URL that points to an image file (i.e. a web address that ends in .jpg, .png, .tif or .gif). Click "OK" and a grey box with black arrows appears as the overlay until the target image appears.
(3) Editing Ground Overlays
It is likely the overlay will not (at first) be located correctly.
To edit the overlay, reopen the dialog box by selecting the path in the sidebar (click on it so it is highlighted in light blue) and right-click (ctrl-click on Mac). Select Properties (PC) or Get Info (Mac) at the bottom of the menu.
The green guides can be manipulated to resize and relocate the overlay:
- The
edges can be dragged back-and-forth along one axis
- The
corners can be dragged along two perpendicular axes
- The
center point will move all the guidelines whilst maintaining their relative
positions
- The
diamond can be used to rotate the image
It is often helpful to alter the transparency of an image to aid resizing/relocation. This can be done using a slider in the dialog box.
Note: After resizing/relocation, unless preferred it best to turn the transparency back to fully opaque before closing the dialog box. Transparency can still be adjusted using the slider at the bottom of Places.
(4) USGS Maps as GroundOverlays
The KML to create Ground Overlays can be combined with
another piece of KML called Network Links, to create dynamic overlays that
refresh based on a set time or a change in view.
A number of applications have been created using this capability to import the
archive of USGS maps that is freely accessible via the world-wide-web. Two of
the best are described here:
(i) USGS Topographic Maps by Google Earth Library
Ground Overlays that provide seamless topographic/land cover data for the entire Earth and a USGS Topographic Map overlay for the entire US, including Alaska and Hawaii.
A dynamic ground overlay that refreshes based on viewing location and altitude:
http://www.gelib.com/wp-content/uploads/2009/ng-topo_nl.kml
A clickable grid that allows individual 7.5 minute topo maps to be downloaded as KMZ files, by clicking a link in the description balloons associated with the grid boxes (limited data for Alaska):
http://www.gelib.com/maps/_NL/usgs-topographic-maps.kml
(ii) USGS Quadrangles (QUADS)
QUADS is a free web-based service created by Metzger + Willard, Inc for visualizing USGS quadrangles in Google Earth that provides an easy-to-use framework for retrieving georeferenced PDF topo maps from the USGS Store.
http://www.metzgerwillard.us/quads/
(An explanation of QUADS, the KML file and a white paper with further descriptions can all be found via this link.)
Data available for viewing in GE:
The Overlays folder contains multiple Network Links that access different datasets that can be viewed in Google Earth; Topographic Maps, Orthophotos and Relief Maps.
Data available for download:
The QUADS KML creates a rectangle grid based on your view in Google Earth. Inside each grid box is a Placemark (a solid, colored circle), which can be clicked to view active links to download that quadrangle’s data. Different quadrangle scales are depicted by different colored grids.
Note: After opening the KML you need to check the box (in the sidebar) next to the Search Results Network Link to view the colored grids.
chans...@gmail.com
chans...@gmail.com
On Sunday, November 13, 2011 at 8:53:14 PM UTC-7, Geobrowser wrote:
chans...@gmail.com
On Sunday, November 13, 2011 at 8:53:14 PM UTC-7, Geobrowser wrote:
