Welcome to Project Alidade – my humble effort to develop a surveying system based upon Android mobile phones. so far I’ve managed to build a crude yet effective plane table. From following the RCAHMS website and other books on mapping, I have managed to complete my first ever survey using the device. The alidade is the critical piece and I managed to fashion one out of a strip of wood.
The procedure I adopted was as follows:
1. Place the Android mobile over the centre of the plane table, to make it find satellites. After it begins collecting points, switch on “GPS Averaging” app and wait till the results “settle down” to say 0.2m accuracy.
2. Proceed to measure points using the plane table, then enter them into computer, based on station number, bearing and distance.
The most important point is the first one, I named it station 1, right on the dge of my patio as this had good GPS reception from the Samsung. I noted down the date, time and prime coordinate:
Tuesday 11 June 2013 11:35am
North: 47 deg 52.34
East: 11 deg 41.682
Error = 0.8m
Description: Edge of porphyry terrace in east garden
(measured using GPS Averaging app on phone. I tried the same process with the “Precision GPS” app but the result was oddly different)
First task was to orient the board with north using my old Silva compass, then place the sheet (two A4 sheets stuck together) with the axis point in the middle, around which the alidade will swivel.
The photo shows all my gear lying in the table, with the map taped down. In practice I found the table set to be quite flimsy and need a modern standard one. The manuals say you ought o have a flat surface. This proved impossible with my improvised setup.
As you can see I chose the first station at the dge of the patio. The rationale was to maximise exposure to GPS satellites, and I didn’t pay heed to the convenience of accessing the plane table. Experience through blunders!
The table can swivel also with this setup, so I kept the Silva compass on the table at all times during mapping. I need to ensure it doesn’t move in future.
I began to record points. Initially I thought of recording every object in sight but decided to just concentrate on the house outside walls and garden boundary to start with. I would point the alidade at the feature, line up the end single nail in between the rear two nails as shown below:
However the “pointy” end of the alidade needs a sharper end to mark with a pencil, it is just a simple blunt 90 degree end at the moment. If you look at the drawing sheet I generated from this morning’s work:
Hope it’s clear but I was able to record the angle manually, then the distance to the feature with a tape measure. I just realised that the technique I’ve seen on the RACHMS video isn’t the same as mine, as I don’t have a scale rule (my old GIS company had one – should have nicked it!) nor have I established the scale yet of my map, so I am going to have to dig out my kids old school protractor and guesstimate it.
Ultimately one wiould end up with a data set looking something like this:
Bearing Distance Description
2 4.73 House corner
5 deg x Corner of garden wall – distance not measured – I forgot!
237 x Angle only. Corner of garden common with neighbours
275 2.92 House corner
314 5.29 House corner
As I had only drawn the angles on the paper I had to measure the angle using my daughter’s protractor to the nearest degree! This is fiddly and a less than satisfactory result. At he very least I will use the angle bearing measured in the mobile phone app “Smart Compass”.
One thing I didn’t do was to fix points with pegs in the ground.
We have to consider with what we will plot the pionts gathered. Will it be Microstation, Sketchup? What CAD package can draw the map for us?
What is the future for such an app? Ultimately my dream is to have the app sitting atop a laser and small telescope recording a single GPS station point accurately, while a laser fires a weak beam at a building corner, all the while the sport the beam falls can be observed with a small telescope. The app can sit on a small holder on the top. I’m initially going to frame the three devices in wood, but the dream might be to encase it in plastic then put that on a camera tripod.
Have I just “invented” an Android theodolite?