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Jul
29
comment Defining the azimuthal equidistant projection of an Environment Canada weather radar map
@whuber thank you for this useful caution. Why then would my graphical intuition be incorrect: examining the map above there have been 6 concentric circles spaced at 40 km added by the cartographer. The sixth from the centre is tangent with the map borders, implying that the borders of the map are 240 km from its centre. Understanding that a degree of latitude and longitude are not equidistant, why is not sufficient to separately find the latitude that is 240 km north of the centre point, and the longitude that is 240 km west of the centre point in order to find the top left corner?
Jul
28
accepted Defining the azimuthal equidistant projection of an Environment Canada weather radar map
Jul
28
comment Defining the azimuthal equidistant projection of an Environment Canada weather radar map
Assuming this map is true in its alignment, then the coordinates of the corners can be found by adding or subtracting 240 km of latitude or longitude given the centre point.
Jul
28
comment Defining the azimuthal equidistant projection of an Environment Canada weather radar map
A critical and very useful piece of information is evident upon zooming into the 40 km scale bar on the right. Each pixel in this GIF is square representing 1 km in dimension. It is therefore trivial to determine the corners of the map for georeferencing. The centre is at (240,240) in the GIF coordinate system, so each corner is therefore 339.4 km distant from the known longlat coordinates at the centre of the map.
Jul
28
comment Defining the azimuthal equidistant projection of an Environment Canada weather radar map
This is very useful for defining the projection @mkennedy. What would be an efficient way to determine the coordinates of the corners of the GIF, i.e. to georeference the GIF. All I have is the scale bar on the right and,the longlat of the centre point. Features (i.e lakes) with known locations are poorly depicted. Presumably the dimensionality in km of each raster cell could be derived from the scale bar, and then the corners georeferenced using the Euclidean distance in km converted from the raster distance. Is there a more efficient way with ArcGIS?
Jul
28
revised Defining the azimuthal equidistant projection of an Environment Canada weather radar map
added 41 characters in body
Jul
28
asked Defining the azimuthal equidistant projection of an Environment Canada weather radar map
Sep
24
awarded  Autobiographer
Jul
2
awarded  Curious
Feb
21
awarded  Popular Question
Mar
3
awarded  Yearling
Jul
4
accepted Correct use of the terms geographic, path, and Euclidean distance
Jul
4
comment Correct use of the terms geographic, path, and Euclidean distance
Utterly comprehensive and detailed answer! I thought that cost distance calculations were typically implemented using graph theory representations of rasters. The cost distances are, then, found as the minimum weight path through a weighted graph. While this may be a typical algorithmic solution, is the approach you imply likely to be more efficient?
May
17
comment Opening ArcGIS layer package in open source software?
Very helpful. As indicated in your figure, the shapefiles I was looking for were in commonData\Data0
May
17
accepted Opening ArcGIS layer package in open source software?
May
16
asked Opening ArcGIS layer package in open source software?
Apr
30
awarded  Commentator
Apr
30
comment How to intersect lines and polygons in R?
@Simbamangu it doesn't seem that gIntersection() will really do what you want. If your lines, forming a grid, were polygonized, it might work, but this is probably not what you want. I suggest asking this either on StackOverflow where more spatial R folks hang out, or on the R-sig-geo list where you'll reach the architects of rgeos and related tools.
Apr
24
accepted What are some examples of model-based upscaling of raster data?
Apr
22
comment How to intersect lines and polygons in R?
@Simbamangu I just used your files to run the above script, and I found that the line should be changed to: poly <- readShapePoly("poly.shp"). There are no files polygon.* in the zip you provided. It then worked for me, performing the intersection as expected. Could the error be as simple as loading the wrong shape file? (I am using rgeos package version 0.2-1 under R 2.14.2)