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Scaling x,y coordinates in proj4?

0

This question was asked before on Stack Overflow.

Let's say we have mercator map with given resolution (for simplicity in centimeters).

Now let's get coordinates of (lat,lon) = (50, 15):

$ proj +proj=merc +ellps=WGS84

50 15

5565974.54      1678147.52

Since we don't know the boundaries, we can't scale it to desired resolution.

The only way I've found to get boundaries is compute the coordinates for (lat, lon) = (180, 0) and (0, 90), but that's more like a hack and it's not universal (other projections might map only from (±90, ±90)).

Does proj4 deal with scaling differently?

coordinate-system coordinates proj

share|improve this question

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I am confused as to what you are really asking, but from your response to Mintx's answer below, perhaps this post will help with the transformation misunderstanding: stackoverflow.com/questions/2651099/… The accepted answer has a very good breakdown of going from Lat Lon to pixels via Web Mercator.
  
  – evv_gis
  Mar 21 '14 at 13:26
  

  
  
  
  

add a comment | 

0

This question was asked before on Stack Overflow.

Let's say we have mercator map with given resolution (for simplicity in centimeters).

Now let's get coordinates of (lat,lon) = (50, 15):

$ proj +proj=merc +ellps=WGS84

50 15

5565974.54      1678147.52

Since we don't know the boundaries, we can't scale it to desired resolution.

The only way I've found to get boundaries is compute the coordinates for (lat, lon) = (180, 0) and (0, 90), but that's more like a hack and it's not universal (other projections might map only from (±90, ±90)).

Does proj4 deal with scaling differently?

coordinate-system coordinates proj

share|improve this question

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I am confused as to what you are really asking, but from your response to Mintx's answer below, perhaps this post will help with the transformation misunderstanding: stackoverflow.com/questions/2651099/… The accepted answer has a very good breakdown of going from Lat Lon to pixels via Web Mercator.
  
  – evv_gis
  Mar 21 '14 at 13:26
  

  
  
  
  

add a comment | 

This question was asked before on Stack Overflow.

Let's say we have mercator map with given resolution (for simplicity in centimeters).

Now let's get coordinates of (lat,lon) = (50, 15):

$ proj +proj=merc +ellps=WGS84

50 15

5565974.54      1678147.52

Since we don't know the boundaries, we can't scale it to desired resolution.

The only way I've found to get boundaries is compute the coordinates for (lat, lon) = (180, 0) and (0, 90), but that's more like a hack and it's not universal (other projections might map only from (±90, ±90)).

Does proj4 deal with scaling differently?

coordinate-system coordinates proj

share|improve this question

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

$ proj +proj=merc +ellps=WGS84

50 15

5565974.54      1678147.52

share|improve this question

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

share|improve this question

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

edited 8 mins ago

PolyGeo♦

53.8k1781245

edited 8 mins ago

PolyGeo♦

53.8k1781245

asked Mar 20 '14 at 20:53

RobertRobert

11

asked Mar 20 '14 at 20:53

RobertRobert

11

1 Answer
1

active

oldest

votes

3

I think you may be confusing Mercator XY with the image pixel XY.

The Mercator XYs you are seeing in proj4 are derived from the Mercator formula where the intersection of the Equator and Prime Meridian is 0,0. Those Mercator XYs have nothing to do with the scale of your map.

If you're looking to convert lat/longs to the XY pixel location of an image, see the answer to this question here.

share|improve this answer

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think i'm not confusing them, I want to know how to convert between them! Because mercator is rectangular, there must be some linear transformation from coordinates in mercator and image raster with given resolution. Something like (x,y) = (x0 * scaleX + width/2, y0 * scaleY + height/2).
  
  – Robert
  Mar 21 '14 at 0:57
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Mercator is rectangular, but the UOM isn't consistent across the map. X and Y distance increases as you get closer to the poles. (That's why Greenland looks like it's the size of Africa but it's actually about 1/3 the size of Australia) Edited my answer to point to the formula you may be looking for.
  
  – Mintx
  Mar 24 '14 at 23:58
  
  
  

  
  
  
  

add a comment | 

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3

I think you may be confusing Mercator XY with the image pixel XY.

The Mercator XYs you are seeing in proj4 are derived from the Mercator formula where the intersection of the Equator and Prime Meridian is 0,0. Those Mercator XYs have nothing to do with the scale of your map.

If you're looking to convert lat/longs to the XY pixel location of an image, see the answer to this question here.

share|improve this answer

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think i'm not confusing them, I want to know how to convert between them! Because mercator is rectangular, there must be some linear transformation from coordinates in mercator and image raster with given resolution. Something like (x,y) = (x0 * scaleX + width/2, y0 * scaleY + height/2).
  
  – Robert
  Mar 21 '14 at 0:57
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Mercator is rectangular, but the UOM isn't consistent across the map. X and Y distance increases as you get closer to the poles. (That's why Greenland looks like it's the size of Africa but it's actually about 1/3 the size of Australia) Edited my answer to point to the formula you may be looking for.
  
  – Mintx
  Mar 24 '14 at 23:58
  
  
  

  
  
  
  

add a comment | 

3

I think you may be confusing Mercator XY with the image pixel XY.

The Mercator XYs you are seeing in proj4 are derived from the Mercator formula where the intersection of the Equator and Prime Meridian is 0,0. Those Mercator XYs have nothing to do with the scale of your map.

If you're looking to convert lat/longs to the XY pixel location of an image, see the answer to this question here.

share|improve this answer

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think i'm not confusing them, I want to know how to convert between them! Because mercator is rectangular, there must be some linear transformation from coordinates in mercator and image raster with given resolution. Something like (x,y) = (x0 * scaleX + width/2, y0 * scaleY + height/2).
  
  – Robert
  Mar 21 '14 at 0:57
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Mercator is rectangular, but the UOM isn't consistent across the map. X and Y distance increases as you get closer to the poles. (That's why Greenland looks like it's the size of Africa but it's actually about 1/3 the size of Australia) Edited my answer to point to the formula you may be looking for.
  
  – Mintx
  Mar 24 '14 at 23:58
  
  
  

  
  
  
  

add a comment | 

I think you may be confusing Mercator XY with the image pixel XY.

The Mercator XYs you are seeing in proj4 are derived from the Mercator formula where the intersection of the Equator and Prime Meridian is 0,0. Those Mercator XYs have nothing to do with the scale of your map.

If you're looking to convert lat/longs to the XY pixel location of an image, see the answer to this question here.

share|improve this answer

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

share|improve this answer

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

edited May 23 '17 at 12:39

Community♦

1

edited May 23 '17 at 12:39

Community♦

1

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425

answered Mar 20 '14 at 21:57

MintxMintx

4,9621425