topic raster calculator in ArcGIS Spatial Analyst Questions

Spatial - Map Algebra and Raster Calculator

SteveKopp — Tue, 10 Nov 2009 01:54:30 GMT

Several threads have touched on the changes to the Spatial Analyst Map Algebra and disappearance of the Raster Calculator. For all you who love Map Algebra and the Raster Calculator, the team agrees with you! The changes we have made to the Map Algebra language to make it integrate well with Python do change the language, but we feel it still looks and behaves enough like Map Algebra that you???ll feel the benefit is worth it.

There are two Map Algebra experiences, the scripting experience and the raster calculator experience. Improvements to the language were made to improve the scripting experience. Having Map Algebra tightly integrated with our native scripting environment, Python, has not been possible except in Workstation when we wrote our own verison of both. Python provides a rich set of capabilities for you to take advantage of and integrate with our out of the box tools. We believe that Python will be the language of GIScience and this integration will play a key role. A feature you will see in beta2 that compliments this is the ability to move blocks of pixels back and forth between ArcGIS rasters and Python arrays, allowing you to write your own raster analysis tools.

The other side of Map Algebra is the raster calculator experience. We know that the Python window is not a substitute for the point and click experience and direct output of the raster calculator. We plan to build a calculator-like tool that will provide the point and click experience but also be something that you can drop into a model. Envision a combination of the old raster calculator and the SOMA tool. Unfortunately it won???t make it into beta2.

And while I have your attention I???d like to suggest that if you have colleagues who seldom use Spatial Analyst, don???t make them learn Map Algebra. Show them where to find the tools and run it from the dialogs, or better yet build them a little ModelBuilder model with a couple parameters exposed and hide all the details.

Steve

Spatial Analyst -- Map Algebra: Color2hue does not work

WentaoChe — Tue, 10 Nov 2009 04:03:02 GMT

I have used the following Algebra in 9.2, it worked perfectly.
h_grd = color2hue(tmp_grd, rgb.txt)

But it did not work in 9.3.1. like this:
gp.MultiOutputMapAlgebra_sa("h_grd = color2hue(tmp_grd, rgb.txt)")
I can still make it work in 9.3.1 ArcInfo GRID

How to make it work in 9.4? I appreciate your help.

I use GRID daily, but I want to shift my AML to Python, and this color2hue stops my work.

Map Algebra: Color2hue works Now!

WentaoChe — Wed, 11 Nov 2009 23:46:52 GMT

I wrote in the last post that gp.MultiOutputMapAlgebra_sa("h_grd = color2hue(tmp_grd, rgb.txt)") does not work. Now I know the reason.

I need to set absolute path for the output grid like this in Python:

inpath = "d:\\test"
h_grd = inpath + os.sep + 'h_grd'
gp.MultiOutputMapAlgebra_sa(h_grd + " = color2hue(tmp_grd, rgb.txt)")

This is also true in 9.4.

I am glad to know that gp still works in 9.4.

Whats Coming in Beta 2

TedCronin — Thu, 12 Nov 2009 05:22:24 GMT

Steve -

Whats Coming in Beta 2 for python and gp?

Python at Beta 2

DavidWynne — Fri, 20 Nov 2009 23:00:50 GMT

Hi Ted,
For Python, continued fixes and polishing the experience. There are a couple of things that I'd like to specific note:

Exception classes for ArcPy: Dale wrote about these on the arcgisscripting side on the geoprocessing blog a year ago, but they've always been a little hidden. They also were not exposed on arcpy with Beta 1. At Beta 2, they'll be part of arcpy and more obvious.

Steve touched on this, but also new functions that will allow you to convert rasters to and from numpy arrays. These functions should really open up a lot of modeling opportunities.

-Dave

Cool

TedCronin — Mon, 23 Nov 2009 16:08:39 GMT

Yep, I remember that post, just did not read it, saw try except (which I use), and moved on, but I see now about the result objects, so I will need to re read, whats happening on the blog side of gp. Perhaps a blog post about all the new MB iterators would be nice. C'mon Ash.

Bug in map algebra?

CamPatterson — Thu, 03 Dec 2009 20:25:49 GMT

import os,sys,string
import arcpy
from arcpy.sa import *

# this throws an error
>>> outraster1 = Raster('DEM2_Sept09') - Raster(1)
Runtime error <type 'exceptions.RuntimeError'>: HRESULT(E_FAIL): Unspecified error

# this works
>>> outraster1 = Raster('DEM2_Sept09') - 1

#this crashes ArcMap
>>> outraster1 = Raster(1)

Spatial - Map Algebra and Raster Calculator

RyanDeBruyn — Wed, 16 Dec 2009 15:55:26 GMT

Thank you for your post. Please note that this is a known issue and currently a change request as been submitted. It will be fully supported for 9.4 final.

Casting a constant value in a raster object is similar to creating a constant raster with a defined cellsize and extent.

When used in conjuction with a spatial tool you do not need to cast the number as a raster because the properties of the raster are determined from the output from the tool and the value is simply added to the result.

out = Slope(input) + 1

To create a raster from a constant it will require the user to specify the environment cellsize and extent or an error will occur.

arcpy.env.cellsize = 1
arcpy.env.extent = "0 0 100 100"
out = Raster(1)

-Ryan DeBruyn
ESRI Spatial Analyst Team

Spatial - Map Algebra and Raster Calculator

curtvprice — Tue, 22 Dec 2009 22:37:32 GMT

Hi Ted,
For Python, continued fixes and polishing the experience. There are a couple of things that I'd like to specific note:

Exception classes for ArcPy: Dale wrote about these on the arcgisscripting side on the geoprocessing blog a year ago, but they've always been a little hidden. They also were not exposed on arcpy with Beta 1. At Beta 2, they'll be part of arcpy and more obvious.

-Dave

That blog post was very helpful. I used it to build a little python module to help developing script tools, so now with a small template set up I can easily have my scripts tell me where they fail.

Hoto usa Map Algebra in model?

ThomasHack1 — Wed, 23 Dec 2009 08:04:38 GMT

Hi there,

since the Map Algebra Toolset is empty in 9.4 beta I wonder how I can use any Map Algebra in a model without having the SOMA Tool?

Regards

Thomas Hack

How to use Map Algebra in a model?

EricRice — Wed, 23 Dec 2009 16:35:16 GMT

Thomas,

If you note what Steve said in the first post to this thread you will find info regarding your question. You will have to wait until post beta 2.

"We plan to build a calculator-like tool that will provide the point and click experience but also be something that you can drop into a model. Envision a combination of the old raster calculator and the SOMA tool. Unfortunately it won�??t make it into beta2."

Regards,

Spatial - Map Algebra and Raster Calculator

curtvprice — Wed, 23 Dec 2009 17:33:21 GMT

I wonder how I can use any Map Algebra in a model without having the SOMA Tool?

I think it's also really good news that we can still get to SOMA (or MOMA if we REALLY need it, say for $$ROWMAP,$$COLMAP) through the old arcgisscripting object.

In ModelBuilder, you could use Calculate Value to run a python script (or use a script tool) to access SOMA/MOMA -- in the unlikely 😉 case that the new "calculator tool" doesn't do what you need at 9.4.0.

So long Raster Calculator

GaryHecox — Mon, 01 Feb 2010 21:49:04 GMT

Regardless of what is in the inital post to this thread, based on the 9.4 b2 help menu and the reply back from the ERSI support people today on my 9.4 Beta 2 comments, it appears that the easy to use and powerful Raster Calculator is now gone from ArcGIS 9.4/10. The response has been very clear that Raster Calculator has been removed from ArcGIS and replaced by Python Map Algebra scripting. This is a major loss in the capabilities of ArcGIS to analyze rasters and perform raster math. What used to take a few minutes with the Raster Calcualtor now will take much longer as people have to try and stumble through the issues associated with Python scripting. I wish ESRI would understand that the vast majority of GIS analysts are not and never will be computer programmers. They are hired to perform spatial analysis and create maps, not write computer code. The disappearance of Raster Calculator in 9.4/10 is a huge loss for us.

raster calculator

curtvprice — Mon, 01 Feb 2010 23:35:56 GMT

The jury is out - raster calculator has some huge shortcomings in terms of validation and its lack of integration with the rest of the geoprocessing environment.

I do hope the raster team can post a video or an early patch of the graphical interface to create command line map algebra, or a video showing how raster calculator type stuff can be done in the python command line window.

With the intellisense etc I would think it may be *easier* to use than the raster calculator. We will see though.... I need to play with it a bit before I decide whether this is a net negative.

The biggest issue is the loss of a map algebra tool within modelbuilder, but there are workarounds.

May I suggest you post a use-case of a raster calculator expression you would normally do and maybe someone from the raster team can walk you through how to do it in 9.4?

command line map algebra - overwrite?

curtvprice — Tue, 16 Feb 2010 17:41:02 GMT

When I do this at the ArcMap command line (9.4b2):

>>> atanb6  = Ln(upareag + 1) / Tan((Raster("nedslp") + 1) * (3.14 / 180.))

I get a layer in my map. If I hit the back button and do it again with a slight change (like I am used to in Grid), nothing happens (apparently).

>>> atanb6  = Ln(upareag + 1) / Tan((Raster("nedslp") + 1) * (3.14 / 181.))

I need to change the name of the output:

>>> atanb7  = Ln(upareag + 1) / Tan((Raster("nedslp") + 1) * (3.14 / 181.))

This works - a new layer is created in the map with the output.

I would expect the layer to be re-written with a new result, not nothing to happen. What's going on, and does this deserve an error message of some type?

Spatial - Map Algebra and Raster Calculator

TedCronin — Thu, 25 Feb 2010 18:22:26 GMT

I do hope the raster team can post a video or an early patch of the graphical interface to create command line map algebra, or a video showing how raster calculator type stuff can be done in the python command line window.

Is there a possibility in this happening, because I too would be interested in seeing a prototype.