Calibrating map from Ordinance Survey Ireland web site

I have stitched together a few screenshots from the official Irish Ordinance Survey site online map viewer http://maps.osi.ie/publicviewer. Thus I have a single .png file of a rectangular map area. I have selected four reference points near the corners and recorded their pixel co-ordinates, and used the osi mapviewer to give me the Irish Transverse Mercator co-ordinates of these points, and used their online calcaultor to convert to lat and long using http://www.osi.ie/calculators/converter_index.asp.

I have entered this data manually into OruxDesktop version 2.0.4beta selecting DATUM = Ireland 1965 and REFERENCE = Irish Grid (Geographical) and have created a map output selecting SqLite.

This works with OruxMaps on my Android HTC Desire but the GPS location is not very accurate - it is about 200m wrong. What have I done wrong?

microlit wrote:I have stitched together a few screenshots from the official Irish Ordinance Survey site online map viewer http://maps.osi.ie/publicviewer. Thus I have a single .png file of a rectangular map area. I have selected four reference points near the corners and recorded their pixel co-ordinates, and used the osi mapviewer to give me the Irish Transverse Mercator co-ordinates of these points, and used their online calcaultor to convert to lat and long using http://www.osi.ie/calculators/converter_index.asp.

I have entered this data manually into OruxDesktop version 2.0.4beta selecting DATUM = Ireland 1965 and REFERENCE = Irish Grid (Geographical) and have created a map output selecting SqLite.

This works with OruxMaps on my Android HTC Desire but the GPS location is not very accurate - it is about 200m wrong. What have I done wrong?

Hi, microlit;

That calculator returns ETRF89 GPS Co-ordinates, not Ireland 1965 coordinates.
Try:
1.-select 'grid' coordinates in OruxMapsDesktop, and use Irish Transverse Mercator co-ordinates.

orux

Thanks for prompt reply, but maybe I had problems understanding you?...

I selected 'grid' coordinates in OruxMapsDesktop, and I enter Irish Transverse Mercator co-ordinates rather than lat and long. You did not say how to set DATUM and REFERENCE so I have selected DATUM = Irish 1965 and for REFERENCE I first tried Irish Grid and then tried Transverse Mercator and in both cases I am getting completely wrong lat and long for my home which is one point of reference on my map: my Android device displays the map fine but says the it is 57.342N 0.2659E whereas the correct cords are approx 53.1136N and 6.5513W. This is much much worse than my first try! It is similarly wrong whether I choose Transverse Mercator (and use settings I get from www.osi.ie) or Irish Grid.

So then I tried selecting DATUM = Irish 1965, REF = Irish Grid (with "grid" selected), and entering map Grid Co-ordinates from the www.osi.ie site (thus not Transverse Mercator co-ords). This time it works pretty well! I am getting a fix that is only about 20m out. Is the 20m error inherent in GPS maybe?

microlit wrote:Thanks for prompt reply, but maybe I had problems understanding you?...

I selected 'grid' coordinates in OruxMapsDesktop, and I enter Irish Transverse Mercator co-ordinates rather than lat and long. You did not say how to set DATUM and REFERENCE so I have selected DATUM = Irish 1965 and for REFERENCE I first tried Irish Grid and then tried Transverse Mercator and in both cases I am getting completely wrong lat and long for my home which is one point of reference on my map: my Android device displays the map fine but says the it is 57.342N 0.2659E whereas the correct cords are approx 53.1136N and 6.5513W. This is much much worse than my first try! It is similarly wrong whether I choose Transverse Mercator (and use settings I get from www.osi.ie) or Irish Grid.

So then I tried selecting DATUM = Irish 1965, REF = Irish Grid (with "grid" selected), and entering map Grid Co-ordinates from the www.osi.ie site (thus not Transverse Mercator co-ords). This time it works pretty well! I am getting a fix that is only about 20m out. Is the 20m error inherent in GPS maybe?

Hi;

You have to use a coherent set of data: datum, projection and coordinates with the same datum / projection. The last set used seems OK.

20 meters seems a reasonable error, given that you are calibrating by hand the maps. The GPS error should be less, 3-5 mts.

orux