[GRASS-dev] compare a DCELL and FCELL question

ming_han · January 24, 2021, 4:12am

Hi Everyone

I tried to compare if grids in two rasters are the same, one raster is FCELL and another raster is DCELL. I got different result when I int both raster first before comparing them; and when I float() both raster first before I comparing them

Is there any reason for this?

Thanks
Ming

Micha_Silver1 · January 24, 2021, 8:29am

Is there some reason that you expect the rasters to be the same? Maybe
begin by posting the outputs of `r.info` for both rasters, and what
command you used to compare them?

On Sun, Jan 24, 2021 at 6:13 AM ming han <dustming@gmail.com> wrote:

Hi Everyone

I tried to compare if grids in two rasters are the same, one raster is FCELL and another raster is DCELL. I got different result when I int both raster first before comparing them; and when I float() both raster first before I comparing them

Is there any reason for this?

Thanks
Ming
_______________________________________________
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Ben Gurion Univ
Sde-Boker Remote Sensing Lab
cell: +972 (52) 3665918

ming_han · January 24, 2021, 9:49am

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Here is output of r.info for first DCELL raster

±---------------------------------------------------------------------------+
| Map: cat1_acc_riv@PERMANENT Date: Sat Jan 23 22:58:42 2021 |
| Mapset: PERMANENT Login of Creator: m43han |
| Location: main_working_location |
| DataBase: C:\Users\m43han\Documents\Routing_Prod\Prod000001\grassdb |
| Title: cat1_acc_riv |

Timestamp: none

Type of Map: raster Number of Categories: 19432056
Data Type: DCELL
Rows: 4239
Columns: 9254
Total Cells: 39227706
Projection: Latitude-Longitude
N: 50:52:39N S: 40:16:48N Res: 0:00:09
E: 70:10:39W W: 93:18:45W Res: 0:00:09
Range of data: min = 250752 max = 19432056

Data Description:
generated by r.mapcalc

Comments:
if(isnull(str_r), null(), acc)

±---------------------------------------------------------------------------+
(Sun Jan 24 04:45:38 2021) Command finished (0 sec)

Here is r.info output for second raster
±---------------------------------------------------------------------------+
| Map: cat1_minacc@PERMANENT Date: Sat Jan 23 22:58:48 2021 |
| Mapset: PERMANENT Login of Creator: m43han |
| Location: main_working_location |
| DataBase: C:\Users\m43han\Documents\Routing_Prod\Prod000001\grassdb |
| Title: cat1_minacc |

Timestamp: none

Type of Map: raster Number of Categories: 0
Data Type: FCELL
Rows: 4239
Columns: 9254
Total Cells: 39227706
Projection: Latitude-Longitude
N: 50:52:39N S: 40:16:48N Res: 0:00:09
E: 70:10:39W W: 93:18:45W Res: 0:00:09
Range of data: min = 250752 max = 1.817303e+007

Data Description:
generated by r.stats.zonal

Comments:
r.stats.zonal --overwrite base=“str_r” cover=“cat1_acc_riv” method="\
min" output=“cat1_minacc”

±---------------------------------------------------------------------------+
(Sun Jan 24 04:46:50 2021) Command finished (0 sec)

Thanks
Ming

Micha Silver <tsvibar@gmail.com> 于2021年1月24日周日上午3:29写道：

Is there some reason that you expect the rasters to be the same? Maybe
begin by posting the outputs of [r.info](http://r.info) for both rasters, and what
command you used to compare them?

On Sun, Jan 24, 2021 at 6:13 AM ming han <dustming@gmail.com> wrote:

Hi Everyone

I tried to compare if grids in two rasters are the same, one raster is FCELL and another raster is DCELL. I got different result when I int both raster first before comparing them; and when I float() both raster first before I comparing them

Is there any reason for this?

Thanks
Ming

grass-user mailing list
grass-user@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-user

–
Micha Silver
Ben Gurion Univ
Sde-Boker Remote Sensing Lab
cell: +972 (52) 3665918

Micha_Silver1 · January 24, 2021, 10:10am

I can see that the maximum values are different. Have a look at the
Range values.

What are you trying to do? Maybe there's a simpler approach...

On Sun, Jan 24, 2021 at 11:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

     Many thanks for your reply.
     Here is the command I am using:

     if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

      The str_r is a CELL raster. the result is different when I change it to:
       if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

    Here is output of r.info for first DCELL raster

+----------------------------------------------------------------------------+
| Map: cat1_acc_riv@PERMANENT Date: Sat Jan 23 22:58:42 2021 |
| Mapset: PERMANENT Login of Creator: m43han |
| Location: main_working_location |
| DataBase: C:\Users\m43han\Documents\Routing_Prod\Prod000001\grassdb |
| Title: cat1_acc_riv |
| Timestamp: none |
|----------------------------------------------------------------------------|
| |
| Type of Map: raster Number of Categories: 19432056 |
| Data Type: DCELL |
| Rows: 4239 |
| Columns: 9254 |
| Total Cells: 39227706 |
| Projection: Latitude-Longitude |
| N: 50:52:39N S: 40:16:48N Res: 0:00:09 |
| E: 70:10:39W W: 93:18:45W Res: 0:00:09 |
| Range of data: min = 250752 max = 19432056 |
| |
| Data Description: |
| generated by r.mapcalc |
| |
| Comments: |
| if(isnull(str_r), null(), acc) |
| |
+----------------------------------------------------------------------------+
(Sun Jan 24 04:45:38 2021) Command finished (0 sec)

Here is r.info output for second raster
+----------------------------------------------------------------------------+
| Map: cat1_minacc@PERMANENT Date: Sat Jan 23 22:58:48 2021 |
| Mapset: PERMANENT Login of Creator: m43han |
| Location: main_working_location |
| DataBase: C:\Users\m43han\Documents\Routing_Prod\Prod000001\grassdb |
| Title: cat1_minacc |
| Timestamp: none |
|----------------------------------------------------------------------------|
| |
| Type of Map: raster Number of Categories: 0 |
| Data Type: FCELL |
| Rows: 4239 |
| Columns: 9254 |
| Total Cells: 39227706 |
| Projection: Latitude-Longitude |
| N: 50:52:39N S: 40:16:48N Res: 0:00:09 |
| E: 70:10:39W W: 93:18:45W Res: 0:00:09 |
| Range of data: min = 250752 max = 1.817303e+007 |
| |
| Data Description: |
| generated by r.stats.zonal |
| |
| Comments: |
| r.stats.zonal --overwrite base="str_r" cover="cat1_acc_riv" method="\ |
| min" output="cat1_minacc" |
| |
+----------------------------------------------------------------------------+
(Sun Jan 24 04:46:50 2021) Command finished (0 sec)

Thanks
Ming

Micha Silver <tsvibar@gmail.com> 于2021年1月24日周日上午3:29写道：

Is there some reason that you expect the rasters to be the same? Maybe
begin by posting the outputs of `r.info` for both rasters, and what
command you used to compare them?

On Sun, Jan 24, 2021 at 6:13 AM ming han <dustming@gmail.com> wrote:
>
> Hi Everyone
>
> I tried to compare if grids in two rasters are the same, one raster is FCELL and another raster is DCELL. I got different result when I int both raster first before comparing them; and when I float() both raster first before I comparing them
>
> Is there any reason for this?
>
> Thanks
> Ming
> _______________________________________________
> grass-user mailing list
> grass-user@lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/grass-user

--
Micha Silver
Ben Gurion Univ
Sde-Boker Remote Sensing Lab
cell: +972 (52) 3665918

--
Micha Silver
Ben Gurion Univ
Sde-Boker Remote Sensing Lab
cell: +972 (52) 3665918

neteler · January 24, 2021, 11:58am

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

     Many thanks for your reply.
     Here is the command I am using:

     if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

      The str_r is a CELL raster. the result is different when I change it to:
       if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical "equality" is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, ... )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

ming_han · January 24, 2021, 2:51pm

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

Markus_Metz · January 24, 2021, 3:56pm

Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev

ming_han · January 24, 2021, 4:05pm

Hi Everyone

Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, … ) approach may increase the runtime compare to ‘==’ way?

Thanks
Ming

Markus Metz <markus.metz.giswork@gmail.com> 于2021年1月24日周日上午10:57写道：

Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev

Zoltan · January 24, 2021, 4:12pm

Hi,
Is it important to cross-post this discussion on both dev and user lists?

Regards,
Zoltan

···

On 2021-01-24 17:56, Markus Metz wrote:

Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev
_______________________________________________
grass-dev mailing list
[grass-dev@lists.osgeo.org](mailto:grass-dev@lists.osgeo.org)
[https://lists.osgeo.org/mailman/listinfo/grass-dev](https://lists.osgeo.org/mailman/listinfo/grass-dev)

-- 

=============================================
Zoltan Szecsei GPrGISc 0031
Geograph (Pty) Ltd.
GIS and Photogrammetric Services

Cape Town, South Africa.

Mobile: +27-83-6004028
[www.geograph.co.za](http://www.geograph.co.za)
=============================================

ming_han · January 24, 2021, 4:28pm

sorry for the inconvenience. I was just not sure if it is problem that needs developers to answer.
Best regards
Ming

Zoltan <zoltans@geograph.co.za> 于2021年1月24日周日上午11:13写道：

Hi,
Is it important to cross-post this discussion on both dev and user lists?

Regards,
Zoltan

On 2021-01-24 17:56, Markus Metz wrote:
Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev
_______________________________________________
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[https://lists.osgeo.org/mailman/listinfo/grass-dev](https://lists.osgeo.org/mailman/listinfo/grass-dev)
-- 

=============================================
Zoltan Szecsei GPrGISc 0031
Geograph (Pty) Ltd.
GIS and Photogrammetric Services

Cape Town, South Africa.

Mobile: +27-83-6004028
[www.geograph.co.za](http://www.geograph.co.za)
=============================================
grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev

Markus_Metz · January 24, 2021, 4:32pm

On Sun, Jan 24, 2021 at 5:06 PM ming han <dustming@gmail.com> wrote:

Hi Everyone

Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, … ) approach may increase the runtime compare to ‘==’ way?

The formulas are very simple, I don’t think that differences in runtime can be reliably measured. (De-)compression of the data and the operating system’s file cache have a much stronger influence on the runtime.

Markus M

Thanks
Ming

Markus Metz <markus.metz.giswork@gmail.com> 于2021年1月24日周日上午10:57写道：

Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

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Zoltan · January 24, 2021, 4:34pm

No inconvenience - general list etiquette usually says start with ‘user’ and then move to ‘dev’ if it looks like a SW issue.
But I am NOT lecturing you, so please do not take offense.

Keep well,
Zoltan

···

On 2021-01-24 18:28, ming han wrote:

sorry for the inconvenience. I was just not sure if it is problem that needs developers to answer.
Best regards
Ming

Zoltan <zoltans@geograph.co.za> 于2021年1月24日周日上午11:13写道：
Hi,
Is it important to cross-post this discussion on both dev and user lists?

Regards,
Zoltan

On 2021-01-24 17:56, Markus Metz wrote:
Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

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=============================================
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Geograph (Pty) Ltd.
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Mobile: +27-83-6004028
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=============================================
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Geograph (Pty) Ltd.
GIS and Photogrammetric Services

Cape Town, South Africa.

Mobile: +27-83-6004028
[www.geograph.co.za](http://www.geograph.co.za)
=============================================

Markus_Metz · January 24, 2021, 5:53pm

A note about the usage of GRASS_EPSILON = 1.0e-15:

The range of single precision floating point is about 1.4012984643e−45 to 3.4028234664e38

That means for large numbers, using GRASS_EPSILON will also detect floating point precision limits, not only meaningful differences. For small numbers, GRASS_EPSILON will not detect meaningful differences. GRASS_EPSILON could instead be modified with something like “max(abs(map_A), abs(map_B)) * 1.0e-15” to test for meaningful differences.

Markus M

On Sun, Jan 24, 2021 at 5:32 PM Markus Metz <markus.metz.giswork@gmail.com> wrote:

On Sun, Jan 24, 2021 at 5:06 PM ming han <dustming@gmail.com> wrote:

Hi Everyone

Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, … ) approach may increase the runtime compare to ‘==’ way?

The formulas are very simple, I don’t think that differences in runtime can be reliably measured. (De-)compression of the data and the operating system’s file cache have a much stronger influence on the runtime.

Markus M

Thanks
Ming

Markus Metz <markus.metz.giswork@gmail.com> 于2021年1月24日周日上午10:57写道：

Trying to answer the original question: with a DCELL map “cat1_acc_riv” and a FCELL map “cat1_minacc”, why is “float(cat1_acc_riv) == float(cat1_minacc)” not equal to “int(cat1_acc_riv) == int(cat1_minacc)” ?

int truncates to integer while float converts to single precision floating point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9, “float(cat1_acc_riv) == float(cat1_minacc)” becomes “1.1 == 1.9” whereas “int(cat1_acc_riv) == int(cat1_minacc)” becomes “1 == 1”, thus the results are different.

Another reason for possible differences is that float can only represent max 7 decimal digits. E.g. float(194320567) becomes 194320560 but int(194320567) preserves the value 194320567.

Thus the safest is to cast everything to the type with the highest precision. In this case with FCELL and DCELL, use “double(cat1_acc_riv) == double(cat1_minacc)” or even better the suggestion of Markus N.

Markus M

On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming@gmail.com> wrote:

Hi Markus and Micha

I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.

Thanks
Ming

Markus Neteler <neteler@osgeo.org> 于2021年1月24日周日上午6:58写道：

Hi Ming,

On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming@gmail.com> wrote:

Hi Micha

Many thanks for your reply.
Here is the command I am using:

if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())

The str_r is a CELL raster. the result is different when I change it to:
if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())

Note that numerical “equality” is better tested with a threshold test
against the map pixel difference.
As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.

Hence the test needs to be implemented in a different way, i.e. by
using an epsilon.
Essentially something like this:

if(fabs(map_A - map_B) <= 1.0e-15, … )

In your case (untested):
r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())

See related discussions here: [1], [2] and elsewhere.

[1] Comment by Glynn: https://trac.osgeo.org/grass/ticket/2854#comment:9
[2] Comment by Glynn:
https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html

Best,
Markus

grass-dev mailing list
grass-dev@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-dev