Mapping Toolbox

histr

Create spatial equirectangular histogram

Syntax

[lat,lon,num,wnum] = histr(lats,lons) returns the center coordinates of equal-rectangular bins and the number of observations, num, falling in each based on the geographically distributed input data. Additionally, an area-weighted observation value, wnum, is returned. wnum is the bin's num divided by its normalized area. The largest bin has the same num and wnum; a smaller bin has a larger wnum than num.

[lat,lon,num,wnum] = histr(lats,lons,units) specifies the standard angle unit string. The default value is 'degrees'.

[lat,lon,num,wnum] = histr(lats,lons,bindensty) sets the number of bins per angular unit. For example, if units is 'degrees', a bindensty of 10 would be 10 bins per degree of latitude or longitude, resulting in 100 bins per square degree. The default is one cell per angular unit.

Description

The histr function sorts geographic data into equirectangular bins for histogram purposes. Equirectangular in this context means that each bin has the same angular measurement on each side (e.g., 1º-by-1º). Consequently, the result is not an equal-area histogram. The hista function provides that capability. However, the results of histr can be weighted by their area bias to correct for this, in some sense.

Examples

Create random data:

• lats = rand(4)
  lats =
      0.4451    0.8462    0.8381    0.8318
      0.9318    0.5252    0.0196    0.5028
      0.4660    0.2026    0.6813    0.7095
      0.4186    0.6721    0.3795    0.4289
  
  longs = rand(4)
  longs =
      0.3046    0.3028    0.3784    0.4966
      0.1897    0.5417    0.8600    0.8998
      0.1934    0.1509    0.8537    0.8216
      0.6822    0.6979    0.5936    0.6449
  

Bin the data in 0.5-by-0.5 degree cells (two bins per degree):

• [lat,lon,num,wnum] = histr(lats,longs,2);
  [lat,lon,num,wnum]
  ans =
      0.2500    0.2500    3.0000    3.0000
      0.7500    0.2500    4.0000    4.0003
      0.2500    0.7500    4.0000    4.0000
      0.7500    0.7500    5.0000    5.0004
  

The bins centered at 0.75ºN are slightly smaller in area than the others. wnum reflects the relative count per normalized unit area.

See Also

filterm	Geographic filter for data sets
hista	Spatial equal-area histogram

hista

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