Measure polygon areas in free version of Google Earth

This website provides a neat and flexible way of finding out the area of a KML polygon without needing GE “Pro”: http://www.earthpoint.us/Shapes.aspx

There’s a nice range of output unit options.

 

 

Advertisements

Google Earth measures distances as great circles

It’s nice to see that Google Earth draws and measures lines as great circles – i.e. lines follow the surface of the globe – or geodetically speaking, the spheroid. It’s fairly obvious, really, given the 3-D nature of GE, but it’s not something you take for granted in a GIS, where measurements are generally made on a projected plane of some sort, and the result depends on the projection used.

As a sort of confirmation of this, I measured the distance from the north pole to the equator, the orange line here:

GoogleEarthGreatCircles80

The reported result was 10,001,959 metres. Multiply this by 4 to get a circumference of the earth (a minimum circumference, given the flattened, oblate nature of the spheroid), and you get 40,007,836 m.

Look up the ‘textbook’ value of this circumference, calculated from the equatorial radius and the flattening factor, and you get….. 40,007,863 m.

So, it does indeed look as though Google Earth (a) measures on the spheroid surface and (b) it uses WGS84. all of which is perfectly well known anyway, but it’s nice to confirm it….

ArcGIS: altering display properties of ArcGIS Online services

Sometimes it’s frustrating not to be able to alter the display of basemaps provided by ArcGIS Online. In this case, I was using “World Shaded Relief” (on account of its high, SRTM-derived resolution, but didn’t want its reddish hue. Further, although the transparency of an AGOL layer can be altered, other display properties (brightness, contrast, resampling) cannot be changed.

So this article came  to the rescue: Creating a grayscale version of any ArcGIS Online basemap. It’s somewhat convoluted, involving the creation of a Mosaic Dataset, so I won’t try to repeat the method here. However, it did work, and I was able to alter the colour balance and contrast.

I found a further benefit: after adding the new layer and changing the display as needed, it was then possible to export the data to create a local copy, available for off-line / low bandwidth use.

ArcGIS: copying labelling between layers

It’s easy to copy symbology between layers in ArcGIS (Layer Properties > Symbology > Import > from another layer), but there’s no equivalent for labels. If you’re using “Label all features the same way”, this doesn’t matter much; it’s easy to re-create the label style. But if you’re using several label classes, then re-creating all of the query expressions as well as the label styles isn’t a great prospect. This is an official “ArcGIS idea“, but in the meantime this neat add-in tool, “Import labels from another layer“, does the trick perfectly.

ArcGIS: create features while a definition query applies?

ArcGIS 10.1, trying to add new features to a Shapefile: make it editable; set up an editing template; but the relevant layer refuses to appear in the ‘Create Features’ window. Why?! Well, it seems to be a side-effect of the way that ‘Create Features’ works, explained in an ESRI blog. While editing templates are useful, isn’t this a rather disproportionate level of complication for an everyday operation?

“Web mercator” coordinate system: explained!

I’ve spent quite a lot of time trying to understand the differences between the several flavours of the “web mercator” coordinate system used by many web-based mapping systems to store and present their data. Various names, various EPSG codes, various definitions and various conversions are all in use. So I was pleased to come across this detailed explanation. It’s not a quick read, but it does make sense of a fraught field – thank you to Alastair Aitchison.

Using QGIS for wildlife training in Tanzania

From my trip to Tanzania in February this year:

https://www.qgis.org/en/site/about/case_studies/tanzania_udzungwa.html