Site Map News Organization Search

HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2 -scan

### Introduction:

A grib message contains the grid point values. If you are a C programmer, the grid would naturally be in "C" order. A fortran programmer would have the grid in fortran order. Some people would start from the NW corner and others would start from the SW corner. In grib1, all these scan orders are possible. The grib1 scan orders are,

1. WE:SN
2. WE:NS
3. EW:SN
4. EW:NS
5. SN:WE
6. NS:WE
7. SN:EW
8. NS:EW
You read the above notation (WE:SN) by first go from W to E then go from S to N.
```AB:CD
do i = C to D
do j = A to B
data
enddo
enddo
```
Grib2, includes the above 8 scan orders and adds 4 more useful scan orders.
1. WE|EW:SN
2. WE|EW:NS
3. EW|WE:SN
4. EW|WE:NS
```AB|BA:CD
odd_row=true
do i = C to D
if (odd_row) {
do j = A to B
data
enddo
}
else {
do j = B to A
data
enddo
}
odd_row = not odd_row
enddo
```

This "plow the field" order reduces the file size when using regional grids and saving the increments.

Grib2 has 16 scan orders of which 3 are common: WE:SN, WE:NS and WE|EW:SN. Wgrib2 simplfies life by internally converting fields to WE:SN by default.

```-scan
```

### Example

```-\$ wgrib2 ds_ens.grb -scan
1:0:scan=5 input=WE|EW:SN output=WE:SN
2:218709:scan=5 input=WE|EW:SN output=WE:SN
3:434276:scan=5 input=WE|EW:SN output=WE:SN
...
input=WE|EW:SN   the grib file is in WE|EW:SN scan order
output=WE:SN     the internal registers and output files except for grib
will be in WE:SN order, use the -order option to change
the output scan order
```