Coverage for pyguymer3/geo/add_OSM_map_background.py: 7%

1#!/usr/bin/env python3 

2 

3# Define function ... 

4def add_OSM_map_background( 

5 ax, 

6 midLat, 

7 res, 

8 /, 

9 *, 

10 debug = __debug__, 

11 scale = 1, 

12): 

13 """Add OpenStreetMap map tiles as a background to a Cartopy axis. 

14 

15 Parameters 

16 ---------- 

17 axis : cartopy.mpl.geoaxes.GeoAxesSubplot 

18 the axis to add the OpenStreetMap map tiles as a background to 

19 midLat : float 

20 the latitude of the middle of the figure (in degrees) 

21 res : float 

22 the resolution of the figure (in m/px) 

23 debug : bool, optional 

24 print debug statements 

25 scale : int, optional 

26 the scale of the tiles 

27 

28 Notes 

29 ----- 

30 Copyright 2017 Thomas Guymer [1]_ 

31 

32 References 

33 ---------- 

34 .. [1] PyGuymer3, https://github.com/Guymer/PyGuymer3 

35 """ 

36 

37 # Import standard modules ... 

38 import os 

39 

40 # Import special modules ... 

41 try: 

42 import cartopy 

43 cartopy.config.update( 

44 { 

45 "cache_dir" : os.path.expanduser("~/.local/share/cartopy_cache"), 

46 } 

47 ) 

48 import cartopy.io.img_tiles 

49 except: 

50 raise Exception("\"cartopy\" is not installed; run \"pip install --user Cartopy\"") from None 

51 

52 # Import sub-functions ... 

53 from ..openstreetmap import zoom 

54 

55 # ************************************************************************** 

56 

57 # Calculate the zoom depending on the central latitude and the resolution of 

58 # the figure ... 

59 z = zoom(midLat, res, scale = scale) 

60 if debug: 

61 print(f"INFO: The resolution is {0.001 * res:,.1f} km/px and the OpenStreetMap zoom is {z:d}.") 

62 

63 # Add OpenStreetMap tiles ... 

64 # NOTE: It appears that the background image is drawn at only 72 dpi. If you 

65 # zoom in then there are around 4 pixels in the red lines joining the 

66 # points together for every pixel in the background image (300 / 72 ~= 

67 # 4). Try regenerating a map at 72 dpi and the background image will 

68 # be the same resolution as the red lines joining the points together. 

69 # NOTE: Line 528 of "cartopy/lib/cartopy/mpl/geoaxes.py" is the "imshow()" 

70 # call. 

71 # NOTE: Line 71 of "cartopy/lib/cartopy/io/img_tiles.py" is the 

72 # "image_for_domain()" definition. 

73 # NOTE: Line 588 of "cartopy/lib/cartopy/io/img_tiles.py" is the 

74 # "_merge_tiles()" definition. My only guess is that some pixels are 

75 # not getting sliced correctly when making the merged image because 

76 # you should never do floating-point equalities on line 608. 

77 osm = cartopy.io.img_tiles.OSM( 

78 cache = True, 

79 ) 

80 ax.add_image( 

81 osm, 

82 z, 

83 interpolation = "none", # NOTE: Due to the use of **kwargs 

84 # within Cartopy, this is passed 

85 # all the way down the stack to 

86 # the ".imshow()" call. 

87 )