If the area being mapped has a complex topography, first identify it on google earth pro.
Let's take a look at this site.
Draw the flight boundary such that the edges are about 15-20 meters away from the site boundary edges
This ensures that sufficient number of images capture each GCP target.
Then start marking the GCPs. First, I will mark 5 GCPs -
4 on the edges of the site ..
and one at the centre. Now, with google earth pro you can get a 3D view of the site and identify the elevation variations.
We can clearly see a hill over here
let�s account for this point by marking a GCP there. And this look like a a lower point
so I�m going to mark another one here.
Overall, this looks like a great GCP strategy!
One of our students asked us a question about corridor mapping and how to go about planning the GCPs.
He also provided the lattitude and longitude values of the road extents.
So, let�s jump to Google Earth Pro to show you this example.
We have the road here.
and these are the extents that need to be mapped.
First, let�s measure the length of the road extents
It is 435.18 meters.
Now, since it is a pretty long strip,
let�s use 8 GCPs.
So, 435.18 divided by 8 is approximately 55 meters.
So, let�s identify all the GCP positions.
Let�s place the first one here and call it 'GCP1'
Now, we don�t want all the GCPs in a single straight line
so we�re going to place them in a zig-zag manner.
So, let�s measure the distance to the next GCP which is approximately 55 meters away
and the second GCP goes here..
Similarly, all the remaining 6 GCPs are placed.
Now, if you want to have these points available to you once you are at site,on your google maps application,
save the points and the boundary in a new folder
and save it as a kml file.
Go to google maps.. your places... maps and click on �Create Map�.... Click on �Import� and drag/drop the kml file..
And there you go!
It is now available on Google Maps.
Open your app to access the map and use these points to locate the GCPs at site in relation to where you are
So while at the site, you can approximately move to these positions and place and measure your GCP points .
For whatever GCP position design you choose, you can verify if it is a good one or not
�The Tarpaulin Method�.
Let me explain.
Consider the site you are mapping is this one.
let me draw the boundary for it.
So I draw the lines that define the boundary.
And I finally close it here.
Now imagine the surface of this site to be a tarpaulin sheet which needs to be secured if a breeze blows across it.
And you have weights which you can place on the sheet to secure it.
Where will you place these weights on the sheet?.. I would place them at these points
Ignore the fact that there are trees, for the purpose of this example.
Now if the breeze blows across it, would it remain secure?
Definitely yes!
Now, each GCP is similar to the weight placed on the sheet.
The position of GCPs should be exactly at those points which are corners and one at the center to secure
the sheet from flying off!