I think that might be because there is no single, clear answer. I'll try to summarize my knowledge and the two most related questions:. Let's start by taking Google Earth and even GIS out of the equation and only consider how accurate a measurement on an image can be.
If the resolution of an image is a pixel is 1m, and you are zoomed in far enough to see individual pixels (original, not resampled for display purposes), you can measure to within +/- 1m. Now let's get that into GIS, which requires both orthorectification and georeferencing.
Both can introduce error. On top of which, you're looking at errors introduced by (re)projection.
Now let's put that all into Google Earth, which is providing you a seamless interface to multiple original sources. All of which may vary in their original resolution and quality of the above processes, so right off there is no single accuracy measurement.
The Google Earth can simply put the options for ruler unit in Preferences so that it remembers your choice until you decide to change it. Right now, I am satisfied with what Aspens suggested, but this still leaves other options---meters, feet, centimeters, yards, etc.---out.
This is why Google has that disclaimer about accuracy: Google makes no claims as to the accuracy of the coordinates in Google Earth. These are provided for entertainment only and should not be used for any navigational or other purpose requiring any accuracy whatsoever. Our imagery varies from sub-meter resolution in major cities to 15 meter resolution for most of the earth's surface, with a global base resolution of 1KM.
Since our database is constantly being updated, we cannot state a specific resolution for any geographic region. Google acquires imagery from many different sources with many different file formats, projections and spectral characteristics. All imagery sources are fused into a single global database with a proprietary format that has been developed for the specific purpose of streaming to our client software., as linked in question 2 above. All of that is just for the accuracy of the imagery you're measuring from. Then add in the user interface - how far out are you zoomed when you do your measurements?
Far enough in to see those individual original pixels, if they're even available to you? Or out far enough to see the entire field, or the field and its surroundings? What display resolution is your computer running, and how accurate/precise is your hand with the cursor as you mark the boundary? What distortions are introduced by the projection(s) Google Earth uses, which may change depending on where you are looking. The best you can do is find something of known distance near your feature and see what it measures at to estimate the error in that area.
So the short answer is, it varies. You have assessed the variance in the precision of the value. This is only related to accuracy.
The details of how the 'Measure' tool in GE is not known to me, but I assume it is using some on the fly projection process. If you use data projected in Web Mercator to calculate distances and areas in ArcGIS the numbers are pretty much meaningless. This topic is constantly revisited as it is a readily available tool to share and discuss spatial data. If you really require accurate areas use an area preserving projection designed for your area of interest, not Google Earth. – May 17 '16 at 17:56.
The following are notes on my GTI presentations about how to create Paths in Google Earth. If you have questions please add a reply to this post and I respond as soon as I can (if someone else doesn’t answer first!) -JB (1) What is a Path? A 'path' in Google Earth is a series of vertices tessellated together to form a line. In the KML code is a member of the family. (2) Creating Paths There are a number of ways to add KML objects in Google Earth. The three simplest methods are: (1) Use the Menus (Add - Path) (2) Click on the icon (showing three dots) in the Toolbar (3) Right-click on a folder in the Sidebar and select Add - Path When the dialog box appears move the cursor into the 3D Earth browser, and click on the globe to create vertices.
These will automatically become connected by thin white line segments to create a path. To change the thickness/color/opacity of the Path select options under the 'Style, Color' tab in the dialog box. Paths can also be automatically generated when importing GPS track logs (Tools - GPSand select 'KML LineStrings' as the output) KML Examples – a file created by USGS uses Paths to delineate plate margins – the KML code has been modified so that the blue line tracing the river turns red on mouse-over – an example of a GPS file that includes a KML LineString (Path) (3) Editing Paths To edit a path re-open 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 that appears.
When the dialog box appears the vertices of the path will appear in the 3D browser as square red dots. To Select a Vertex for Editing - Roll the cursor over the chosen vertex (it will turn green) and left-click on the vertex (it will turn blue). To Delete a Vertex – Select a vertex (see above) and press delete (or backspace). To Move a Vertex – Select a vertex (see above). Hold down the left-click and drag the vertex to a new location. To Add a Vertex – Select a vertex (see above) in at the segment of the path to add vertices to, and click in the 3D browser.
If no vertex is selected, clicking in the 3D browser will add vertices to the end of the path. (4) Using Paths (a) Paths can used to create walls-like structures Open the dialog box and select the 'Altitude' tab. In the drop down menu change Clamped to ground to Relative to ground, and make the Altitude a value, e.g., 1000m. (Note: In mountainous terrain not all the path will appear above the landscape if it is not raised high enough) To create the wall, select Extend path to ground. The color of this wall can now be changed under the 'Style, Color' tab (the values for 'Area'). (b) Using Paths to generate automated tours Any path can be used as the guide for a Driving Tour.
Select the path in the sidebar (click on it so it is highlighted in light blue), then press the play button that appears at the end (right side) of the Places panel. To change the settings for this tour open the preferences menu: PC: Tools - Options - Touring (tab) Mac: Google Earth - Preferences - Touring (tab) Change the values listed under When creating a tour from a line. Tours will be discussed more in a separate forum post. KML Examples – An elevated path extended to the ground to create a 'wall' (turn on '3D buildings' to compare with a model of the real Great Wall). – Drive along the National Mall in Washington DC using the tour function. For optimal viewing turn on '3D buildings' and set the tour properties to Tilt Angle=80, Range=200, Speed=200.
– use of KML Tracks and a 3D model (exported from SketchUp) to 'fly' through the Grand Canyon (.Advanced KML.). (5) Measuring Paths The ruler tool (Tools-Ruler or icon in Toolbar) can be used to draw a line that measures the distance between two points or the length of a multi-segment path. These lines can be saved as KML Paths. The length of a path that has been previously created is show in the 'Measurements' tab of the (editing) dialog box.
(Note: this last feature is the opposite to what I stated during the tutorial and will require an up-to-date version of Google Earth) (6) Topographic Profiles A topographic profile – a side profile graph that shows elevation changes along a Path. They are created by selecting a Path (in the Sidebar), right-clicking, and selecting Show Topographic Profile. Topographic Profiles can be generated from any Path, regardless of how the Path was created. KML Examples – an illustration of how topographic profiles can be applied to the flow of rivers and understanding drainage basins (.Note: this is still developmental beta version.) Gary 3/10/2011, 19:20 น.