Georeferencing Aerial Photos, Field Sketches and Trench Maps

[WhiteStarLine]

Georeferencing aerial photos, field sketches and trench map extracts is a powerful way to visualise a WW1 location on a modern map.  You can find trenches, craters and other dispositions and see exactly where they were on a modern map.  The hard part, as always, is finding a reference point in common.  Then, superimposing the century old image on a modern map was painstaking and beyond the reach of most.

 

Today, we release GeoRef, part of the tMapper stable.  This web site allows you to upload an image, crop any margins, white space or unwanted parts, then tag 4 corners or 3 known points.  These can be trench map coordinates or latitude & longitude.  Shortly, a name such as Minden Post will suffice.  If the 4 corners are accurate, then the image overlay is superimposed on a modern map.  If (as usual) the image doesn't quite fit, then we take a best guess and you can finish off with sophisticated utilities that let you drag, rotate, zoom, combo-zoom-rotate and rubbersheet.

 

The hard part is identifying the corners or 3 points on an aerial photograph (only near vertical shots are currently supported).  In our tests, 3 well defined points on a photo or known trench map coordinates for each corner meant that the image was georeferenced within a few minutes.

 

Here is an example, taken from the Tutorial.  A map is loaded from your hard drive to the tMapper server.

 

 

A specific area can be cropped, or the entire trench map extract selected.

 

 

Each corner (preferred) is entered, usually by locating the area covered on tMapper.  Trench references are converted on the fly to lat, lon pairs.  Alternatively, the map has a church, crucifix and old mill that can be tagged and a map reference entered.

 

 

With 4 corners entered, the image is georeferenced and in this example, no further action is required.  With the map zoomed in and the opacity reduced, each road seems to correctly align, with the possible exception of the one on the west face.  There is a shadow of an old road visible - was the modern road built slightly north or is it an error on the original map, or a mistake by the person georeferencing it?

 

If all corners are known, then maps orientated with south upwards or other orientations are correctly aligned (north up).

 

Different baselayers can be selected (aerial, street, topographical) and boundaries and map markers hidden.  An opacity slider makes the modern features under the image visible and a map scale and current latitude & longitude assist with confirmation.

 

 

Issues and limitations are clearly stated in the Tutorial and if you would like to get involved by suggesting improvements, then please feel free.

 

Finally, we can't stress how important it is for a minimum of 3 features to be identified.  There is a famous aerial photograph showing multiple derelict tanks near Ypres.  It has one reference point that can be readily and exactly identified.  However, none of us can georeference that photo!

 

 

 

[WhiteStarLine]

This aerial photograph, courtesy of McMaster University, took under 3 minutes to load and georeference for a first cut.  To finish off, around 5 minutes work is still required to drag the RHS slightly closer to Westouter and then rotate the map a fraction of a degree anti-clockwise so that the roads on the west and south face are better aligned.  The opacity slider is an essential tool for this.  However, as a first cut, based on entering 28.M.9.c.3.5, 28.M.8.c.7.2 and 28.M.14.b.5.2, it is pretty reasonable.

 

These locations were found by comparing the photograph's single reference point, 28.M.14, with this location on tMapper.  Full instructions can be read as part of the tutorial.

 

[WhiteStarLine]

Field sketches can be as precise as a surveyor's tracing, to as sloppy as a field notebook scrawled in the dark in the rain.  This one was drawn by David Jones for In Parenthesis.  Acid Drop Copse, the Quadrangle and a distinct tongue-like rectangle on the eastern side of Mametz Wood give us an idea of where he was.  Georeferencing on a topographical map shows the approximate location of the trenches.

 

Quite some rubbersheeting was necessary to align these 3 points and this has to be taken into account when assessing veracity.  The 3 map markers show the underlying geographical position and the overlying sketch is a reasonable, if skewed match.

 

[TEW]

An impressive tool. Seen in action in the Delville Wood Topic.

 

I can see this tool occupying my time, just need to find the sketch maps and others again.

TEW

Edited 24 September , 2020 by TEW

[WhiteStarLine]

1 hour ago, TEW said:

The georef link doesn't work in your 1st post of above topic.

Sorry, now corrected.  It would have helped if I put http:// instead of http/;/

[TEW]

Will edit my post.

TEW

[WhiteStarLine]

Mine Crater Discovery - Tutorial for Georeferencing Serre Trench Map Extract

 

A forum pal sent me a trench map extract and asked me to georeference it, which I was happy to do.  When we checked a location of 2 mine craters we both noticed they were visible as scarring in the modern landscape, something you'd probably not pickup ordinarily.  So I thought I'd make this a tutorial.  The original extract can be downloaded if you want to follow on.

 

I've also been asked to make the result available so that it can be used on an iPad in the field (ie not only showing a georeferenced map but the GPS displays the current location).  This hopefully will be available in 2-3 weeks.

 

The 1st step is to work out the corners.  This can be the hardest part, depending on how clean the extract is and if it straddles map junctions.  The better the corners are identified, the less manual adjusting at the end.  I used a program such as paint.NET to work out the length of a trench square, then calculated the offset as a fraction.  This gave me corners, which I stored in Notepad:

 

   NW   57d.K.21.c.89.94   NE   57d.L.19.c.05.93

   SE   57d.K.33.c.90.02   SW   57d.L.31.c.04.03

 

 

I opened GeoRef and loaded the trench map.  As I wasn't doing any cropping, I just clicked the 'Tag Cropped Image' button to continue straight on.  I then entered each corner as a trench map reference (lat, lon is also accepted) and clicked the button to georeference.

 

 

The estimate was pretty good but I wanted to stretch it out a fraction to anchor the crossroads in Serre and have the map on the top left overlay the roads.  I use the Free Rotate button for this and move each corner very s l o w l y.

 

After 3 or 4 minor alterations, 9 out of 11 roads aligned really well.  This is as much as I hope for, georeferencing a 100 year old paper map on a modern satellite one.  Here is the top right corner:

 

 

Now you can select 4 types of base map, measure distance and bearing and use the opacity slider to swap between the modern and Great War view.  I was surprised to check the mine two craters 950 m west of Serre and see two distinct impressions in the ground - the lip of each crater still faintly visible despite 100 years of agricultural use.  I also thought the British line still scarred the field, but this could also be an old farm track.  See this for yourself at serre.html.

 

Trench map from @Nick Bradbury