Map and Data Set Coordinates (Grids)

 

When GIS was young and computers had limited power, it was advantageous to limit the use of the projection equations.  This meant that you would reproject a data set at most once, and work with that data set in the same projection as the map.  The USGS traditionally used UTM coordinates for its data sets, leading to challenges on the boundaries.

As computer power grew, the GIS software could reproject on the fly.  This led to a desire for seamless data, with no problems at the edges of the data sets, and geographic coordinates to store the data.  USGS went this route when they rolled out NED and Seamless Server; NGA had always use geographic coordinates. 

Geographic coordinate maps (plat caree) should only be used for global grids when significant distortion cannot be avoided; for large scale maps, there must be differential scaling in x and y.

 

Table 1.  General case

Data  Set, Grid coordinates Data  Set, Projected coordinates Data  Set, Geographic coordinates Map, Projected coordinates Map, screen coordinates
Integer, window into earth Float Float Float Integer, Window into map
DEMs, start 0,0 in SW corner
Satellites, start 0,0 in NW corner
Equal spacing x and y, in projected meters Equal spacing x and y, in degrees, minutes, seconds
Very rarely adjust spacing for tiles near poles
Projected meters Stats 0,0 upper left
  SPCS (TM, LCC)
UTM
AEA
PS
     
  NLCD NED
SRTM
DTED
ETOPO
   
--------------->
Simple translation, scaling
(corner, dx, dy)
--------------->
Data set projection, inverse equations
Expensive to compute
--------------->
Map projection, forward equations
Expensive to compute
--------------->
Simple translation, scaling, rounding
(corner, dx, dy)
Display
  <------------
Simple translation, scaling
Can round, or interpolate
<------------
Data set projection, forward equations
Expensive to compute
<------------
Map projection, inverse equations
Expensive to compute
<------------
Click on map
Simple translation, scaling,

 

 

Table 2.  Least expensive option, boundary problems

Data  Set, Grid coordinates Data set and Map, Projected coordinates Map, screen coordinates
Integer, window into earth Float Integer, Window into map
--------------->
Simple translation, scaling
--------------->
Simple translation, scaling, rounding
Display
  <------------
Map projection, inverse equations
Expensive to compute
<------------
Click on map
Simple translation, scaling,

 

Table 3.  Seamless case using geographic coordinates

Data  Set, Grid coordinates Data  Set, Geographic coordinates Map, Projected coordinates Map, screen coordinates
Integer, window into earth Float Float Integer, Window into map
--------------->
Simple translation, scaling
--------------->
Map projection, forward equations
Expensive to compute
--------------->
Simple translation, scaling, rounding
Display
  <------------
Simple translation, scaling
Can round, or interpolate
<------------
Map projection, inverse equations
Expensive to compute
<------------
Click on map
Simple translation, scaling,