Locating photos - Republic of Ireland

News: For registered Geograph contributors, see forum thread A couple of tools for use with OS Ireland's online mapviewerExternal link. Peter Facey has written two tools to help in looking at maps of the Irish Republic, going straight to OSI online map from an old-style grid reference, and displaying references in that format.

There are four basic methods of obtaining the grid references for subject and photographer positions when submitting photos in the Republic of Ireland. These are use of a 1:50,000 scale paper map, a GPS unit, Google maps on the Geograph submission system, or maps and aerial photos on the Ordnance Survey of Ireland (OSI) website. All give positions correct within 100 metres. However, because there are two main grids and three main mapping databases in use, there are discrepancies between them. For more accuracy or to make sure positions close to a 1km grid line are within the correct square, see more detailed descriptions below. There are other methods, for example online maps of heritage sites, and county planning maps.

The grids and some of the methods are the same for Northern Ireland.

If using Google maps from the submit page be aware of inaccuracy particularly if using the map display rather than the satellite image.

The location given on the OSI map as the pointer is moved around is an ITM (Irish Transverse Mercator) grid reference. This is different from the Irish Grid used by Geograph and on OSI paper maps. An explanation is given in the "Irish Grid and ITM" section below.

Choosing the correct grid square for submission

For a submission to be a geograph, the photographer and subject positions have to be within the same 1km grid square. Using Google maps from the submit page, or using the ITM co-ordinates from the OSI maps without converting to Irish Grid, can put a position over a grid line into the next square.
The simplest advice is to look at the 1:50,000 scale paper map or 1:20,000 scale online map with grid lines marked - these are on the Irish Grid. If clearly within the correct square put in the location as given by Google maps or the OSI online system, tick the box for "Display 6 figure reference". If close to a grid line estimate position from paper or online map with grid lines marked.

Global Positioning System (GPS)

GPS units can generally be set to give Irish Grid references and give readings as 10 figure GRs (1 metre square). Accuracy is usually correct within 10 metres - on some GPS units the accuracy is given as satellites are found. It is recommended that for Geograph purposes references are quoted as 6 figures (100 metre square) or 8 figures (10 metre square).

Paper maps from Ordnance Survey Ireland

OSI publishes Discovery Series paper maps at 1:50,000 scale. These have Irish Grid lines marked at 1km intervals, and an explanation of the National Grid system in the margin. Use a ruler or romer to measure position within a square. The scaled size of some symbols is larger than the physical size, for example main roads appear to be 50 metres wide and buildings beside them may be pushed out of position.

Google maps on Geograph submission page

At Step 2 of the submission process a Google map is displayed with Irish Grid lines superimposed. You can move subject and photographer icons, and their position as 8 figure grid references (GRs) is changed in the submission form, giving the position as a 10 metre square. You can specify to only display 6 figure GRs (100 metre square).

On the map on the submit page, or on the photo page of one already submitted, there are options. Initially an outline map is displayed, with roads, coasts, rivers and names. Buttons for + and - provide zooming in and out. There are buttons for "Sat" to give satellite images, "Hyb" for Hybrid to give an outline map superimposed on a satellite photo, and "Ter" for Terrain which gives 100 metre contour lines with terrain shading, roads, coasts, rivers and names. The "Map" display when first displayed is an outline, after choosing another option and going back to "map" the roads are shown wider. However "Ter" continues to show roads as a narrow outline.

If you display "Hybrid" you will see that the outline map positions of roads can be some distance from their satellite images - for example 30 metres. And positions of coasts and river banks can be substantially away from the satellite images - for example 100 metres. Furthermore the grid lines shown can be several tens of metres away from their real positions. So if using this system it is advisable to click the box on submission for displaying GRs only to six figures - i.e. to 100 metres.

The Google map often shows areas very inaccurately. For example roads can appear straight when in fact they twist. Also the coastline can appear very generalised, taking in islands as part of the mainland or failing to show inlets or promontories at all. Therefore be careful when locating using Google map and if possible get a location from the Google satellite image or the OSI paper or online mapping or aerial photograph.

Ordnance Survey Ireland online maps

OSI supplies online maps and satellite images at various scales, OSI interactive mapExternal link - or from OSI home pageExternal link choose "Try our new interactive map". They include a map at 1:20,000 scale with Irish Grid lines marked at 1km intervals; this appears to be the same graphic as the paper 1:50,000 map with some extra detail of roads and buildings. Zooming goes by stages to a 1:1500 scale aerial survey, with a "Map products" menu giving several display choices:
As the pointer is moved on the map or image the ITM grid reference is given in the bottom left hand corner with a precision of 1 metre. To get Irish Grid use OSI conversion - ITM to Irish Grid etcExternal link.
Using the larger scales, positions can be obtained accurate to within a few metres.

OSI interactive mapExternal link initially displays a map of the whole of Ireland. You can zoom in by clicking on the map. There is also a search available. Entering a word or part word displays matching hits as house or street names. For example "Ferrycarrig" hits include "Ferrycarrig Lodge, Ferrycarrig, Wexford", also Ferrycarrig Drive, Ferrycarrig Green and Ferrycarrig Park in Dublin, and others.


Archaeology maps

Heritage Service Public MapviewerExternal link initially displays a map of Ireland. You can zoom in to large scale satellite images with the 6" Ordnance Survey map superimposed. Archaeological sites are shown. You can search by county, townland or town, then classification of site. This system gives the Irish Grid Reference directly as 6 figure Eastings and Northings.

Ordnance Survey Northern Ireland online maps


OSNIExternal link has online maps. Choose Geohub, register and loginExternal link. You can then zoom in - progressively down to a 1:250,000 scale colour map, then a 1:50,000 scale monochrome map, then outline maps showing buildings and boundaries. Selecting the + for zoom in, clicking at a point zooms in by a factor of two. Often more useful, click to select the point on the map canvas and drag a box over the area you want to zoom in on.

At all scales, the Irish Grid Reference at the pointer is shown below the map, as figures for easting and northing. Within Northern Ireland these are six digit numbers. To convert to grid references with letters for 100 km squares (myriads) as used in Geograph and on OSNI printed maps, take the first digits of easting and northing and replace them by the letters from the table below.

digits|1 and 3|2 and 3|2 and 4 |3 and 3|3 and 4
letter|G |H |C |J |D


For example Carnlough Harbour is at Easting: 328758 Northing: 418086.
First digits of Easting and Northing are 3 and 4, giving letter D.
So this is D 28758 18086 in 1 km square D 28 18, photo D2818 : Carnlough Harbour, Co. Antrim.

It may be easier to look at an outline mapExternal link showing the grid letters:

Irish Grid and ITM

As mentioned above there are two different grids in use in Ireland (North and Republic), the Irish Grid and Irish Transverse Mercator (ITM). OSI and OSNI have announced plans to change from Irish Grid to ITM to go better with GPS measurement - see OSI - migrating to ITMExternal link. The grids have 1 km grid lines which are close but do not coincide - the table below shows the differences, which vary from nothing to around 80 metres in different parts of Ireland.

Note also that on Irish Grid there are two ways of specifying the 100 km square, either as a leading letter or as initial digits on both Easting and Northing. For example a 1 metre grid reference for Wexford Station can be:
On the first system the position of Wexford Station can also be given as being within T0422 (1km square) or T047223 (100 m square) or T04752234 (10 m square).

The location given on the OSI map as the pointer is moved around is an ITM (Irish Transverse Mercator) grid reference. Geograph and OSI paper maps use the Irish Grid. A conversion systemExternal link between them is available on the OSI website under "GPS services".

Here is a table showing the number of metres difference between ITM and Irish Grid at the south-west corner of each ITM myriad. It gives the Irish Grid myriad letter, the ITM myriad number (eg Irish Grid A is ITM myriad 49), then the number of metres that has to be added to or subtracted from the ITM location northing and easting to give an Irish Grid location. You can get conversions from the OSI website as described above, but the table will give you an idea of the difference in each myriad. Where the SW corner of a myriad is outside Ireland it gives a calculated estimate (marked e). Conversions from OSI are marked c.

| A 49 E+04 N+10 c | B 59 E+26 N+08 c | C 69 E+48 N+09 c | D 79 E+69 N+09 c |
| F 48 E+04 N-13 e | G 58 E+27 N-13 c | H 68 E+48 N-13 c | J 78 E+70 N-13 c |
| L 47 E+04 N-35 e | M 57 E+27 N-35 c | N 67 E+49 N-35 c | O 77 E+71 N-34 c |
| Q 46 E+04 N-56 e | R 56 E+28 N-57 c | S 66 E+49 N-56 c | T 76 E+71 N-56 c |
| V 45 E+04 N-77 e | W 55 E+27 N-77 e | X 65 E+48 N-77 e |



Examples

T0123 : Base of round tower, Irish National Heritage Park by David Hawgood
Click on the thumbnail and look at the map on the image page - the subject position is shown in the middle of the River Slaney. Change to "Sat" and the aerial image shows that the tower is on a promontory.

T0315 : Tractor and manure cart, Piercestown by David Hawgood
Click on the thumbnail, look at the map, click "Hyb" to show hybrid view with map superimposed on aerial view. Click + a few times to zoom in - you will see that the map of the roads does not coincide with the aerial image.

T0018 : Ruin of windmill near Mullanour by David Hawgood
This could be located using the Heritage Service Public MapviewerExternal link.
Maybe the easiest method is to zoom in on the map.
Another way is to put in search terms. Using Wexford as county, Hayestown Great as town, Windmill as classification gets straight to a map superimposed on an aerial photo. The search terms take some getting used to - for example there are many classifications of castle.

More examples where Google maps don't show coastal features
Here are a number of photos where the Google map and its satellite equivalent differ significantly.
M2522 : East of An Trá Bhán (White Strand) by Graham Horn The Google map suggests no land in this square, which there clearly is on the satellite image.

L9622 : Caorán na gCeac Theas (Keeraunnagark South) by Graham Horn The Google map fails to show the large bay, which is clearly seen on the satellite and on the picture.

L9626 : Cuan Chasla (Cashla Bay) by Graham Horn The Google map does not show any land in the northern part of the square. The satellite image does, but even that does not convey the extent of rocky bits seen at low tide.

L8121 : Signal tower on Gólam (Golam) by Graham Horn The Google map does not show this westerly island Golum at all. Even on the satellite it is impossible to make out that the signal tower is in L8121, so only the OSI paper map or OSI website can confirm that.

L8528 : Inis an Ghainimh (Inchaghaun) by Graham Horn Another example where the island is not shown at all on the Google map. It can be seen on the satellite.

Acknowledgement

Graham Horn has helped with suggestions for the text and the examples.
Creative Commons Licence [Some Rights Reserved]   Text © Copyright May 2009, David Hawgood; licensed for reuse under a Creative Commons Licence.
With contributions by Adrian Cable and Penny Mayes. (details)
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