Slingshot view
From SnapMap
A slingshot view is a view tag that defines camera position, camera heading, and horizontal field of view — all in one tag.
It consists of three lat-longs, called position, left and right. From these, as well as camera location (expressed explicitly by the position lat-long), we can easily derive the camera heading and the field of view. It is ideal for photos taken close to the ground where the left and right alignment points can fairly easily be marked on a map. (Note: aerial photos are better tagged with PhotoOverlay or point-match views.)
- position is the location of the camera
- left is aligned with the photograph’s left edge — it can be placed anywhere along the line that radiates from the camera
- right is aligned with the photograph’s right edge — it can be placed anywhere along the line that radiates from the camera
The slingshot view says nothing about elevation or roll, so it works best for photos that are aimed approximately horizontally.
These views are easy to create on top of a Google Maps satellite (aerial photography) view. Aerial photography is usually better than a map as there are often many unmapped features that one can match between photo and imagery, making placement quicker and more accurate. Slingshot views made on top of Google Maps’ map view or OpenStreetMap are useful mainly when the aerial imagery is poor.
Slingshot views are highly recommended for photos taken near ground level.
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Step-by-step guide
- On a SnapMap image page, from the view selector, choose slingshot view.
- Navigate the map until it’s roughly in the position of the photo, zoomed at least so that buildings and roads can be made out.
- Click on the map 3 times to place left, right, and position in their initial locations; don’t worry about accuracy yet.
- Drag the points around until you’re happy the 3 points’ locations are sufficiently accurate.
- Click Add tag. That’s it!
Tips
- Remember that the left and right points can be at wildly differing distances from the camera: place them at whatever locations you find that best align with the photo edges.
- Use the map’s zoom controls to help fine-tune position.
Example
Here's an example, which was set up on top of Google Maps satellite view:
<view type="slingshot" position="51.45303414555948,-2.6009273529052734" left="51.453261460082146,-2.601509392261505" right="51.453619143877546,-2.60125994682312" />
Deleting and editing slingshot views
To delete a slingshot view, find it in the list of tags (the key will be 'view' and the value will start with '<view type="slingshot"...') and click Delete. You can only delete tags you've added yourself, so you cannot delete someone else's slingshot view.
Currently there is no UI for directly editing a slingshot view. Delete the old view and add a new view. Or better, add the new one first – you’ll be able to position it in relation to the old one; then delete the old one.
Why the name?
The reason for the name 'slingshot' is due to the way one marks out these views. The quickest way seems to be first to mark left (L) and right (R) --- they turn out to be easier to position than the camera position (P). Having placed L and R, the P marker can be "pulled back", between L and R like the cradle of a slingshot. The line that projects out from P, bisecting L and R, along the centreline of the photo, helps with P's placement, and also intensifies the metaphor, as if to suggest one is aiming a projectile at the centre of the image.
Notes
Slingshot views have the tag predicate view.
