I need to digitize a bunch of islands from imagery in QGIS. It is the Bing Maps aerial imagery layer. I will obtain all required permissions. My plan is to lock the map scale at 1:25,000 and use heads up digitizing. If it matters I am on the equator.
Assume I do the best job I can do digitizing. I can tell from features I can depict in the map that the imagery is far less than 12.5 m resolution which according to Tobler (table below) would be required to make the 1:25,000 claim. For example, I know from field visits those lakeside umbrellas are only ~2 m in diameter, some of those dirt trails are only ~2 m wide, and the dock is only ~ 1 m wide.
I used the basic Tobler equation below to state that at 2 m resolution (assuming 1 pixel to one cell) I can claim that my scale would be 1:4,000.
I think I am good on (1) as when I lock the scale at 25,000 I can still see clearly depict features far below 12.5 m on the ground, for example, some of the white line widths on the image below.
In many ways it is the opposite of this question. Digitized Map To Scale
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Assume I do the best job I can do digitizing. I can tell from features I can depict in the map that the imagery is far less than 12.5 m resolution which according to Tobler (table below) would be required to make the 1:25,000 claim. For example, I know from field visits those lakeside umbrellas are only ~2 m in diameter, some of those dirt trails are only ~2 m wide, and the dock is only ~ 1 m wide.
I used the basic Tobler equation below to state that at 2 m resolution (assuming 1 pixel to one cell) I can claim that my scale would be 1:4,000.
Similarly, if you need to find out the mapping scale from a known imagery resolution you can do so using the formula below: Map Scale = Raster resolution (in meters) * 2 * 1000.
So, if I digitized at one pixel per cell I could make the claim than my map scale was at least 1:4000 but what claim can I make if I set the scale of the map to 1:25,000? My concerns are:
- Background pyramids serving up a resolution that goes beyond 12.5 m.
- The system used by QGIS to "chuck" cells onto pixels.
I think I am good on (1) as when I lock the scale at 25,000 I can still see clearly depict features far below 12.5 m on the ground, for example, some of the white line widths on the image below.
In many ways it is the opposite of this question. Digitized Map To Scale
أكثر...