Google Aerial Photos

[Red Adder]

I use Google aerial photos to help create base maps but I find that if I zoom in to get good detail I need to stitch together several screen dumps to cover the whole area. Then I get a problem in that whilst I can get one corner of the map to align the others will not - so I would end up with a base map I don't trust.

Do others have a similar problem or do I need to do more that just cut and paste. I find that Freshlogic Studios work fine everytime, though the photos are not always as new as Google.

(As an aside Google now has a history bar which means you cn serach back in time and often get the area in differnt seasons (useful to see under deciduous woods in Winter - though).

[Angry Haggis]

Google and Bing aerial imagery is generally projected in "Spherical Mercator" (aka Web Mercator or EPSG900913) which is not quite the same as British National Grid or Magnetic North. For small areas (i.e. orienteering maps) you can get away with rotating the image. For London projects, I've been rotating the images by around 2 degrees anticlockwise to convert the imagery to British National Grid, which is what my other basemaps have been based on, so that it lines up. The variation is different for different parts of the country. Trial and error is probably the best way.

Incidentally OpenOrienteeringMap also uses Spherical Mercator, so if you use this as a basemap for informal mapping, you will need to perform similar adjustments.

[Red Adder]

I have no problems rotating the final base map to magnetic north - its getting all the photo excerpts to join up - I suspect its the projection they use thats causing me the problem - even over distances of < 200 m.

[Wayward-O]

Google Earth imagery is manipulated to impart perspective using the eye alt and height information. It is also too easy to change the scale and North alignment of the image.

I use the same image data through Google Maps (click 'Satellite' and unclick 'Show labels') and stitch together in Photoshop. Advantages are that the image data is presented at standard enlargements, aligned, unadjusted for a nominal altitude of vewpoint and stitches together perfectly.

For determining magnetic declination corrections, I use:
http://www.threelittlemaids.co.uk/magdec/index1.html

[Red Adder]

Thanks Wayward - that works fine though I doesn't give the choice from historical archieves of Google Earth and for me is a lot slower than Freshlogic - but its free and saves shedloads of time pacing out paths and boundaries so we shouldn't moan too much.