Tag Archives: raster

Classifying Images in ArcGIS for Desktop 10.4

With the addition of the Train Random Trees Classifier, Create Accuracy Assessment Points, Update Accuracy Assessment Points, and Compute Confusion Matrix tools in ArcMap 10.4, as well as all of the image classification tools in ArcGIS Pro 1.3, it is a great time to check out the image segmentation and classification tools in ArcGIS for Desktop. Here we discuss image segmentation, compare the four classifiers (Train Iso Cluster Classifier, Train Maximum Likelihood Classifier, random trees, and Support Vector Machine), and review the basic classification workflow. Continue reading

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Gathering raster properties for multiple datasets using Python

Raster Properties

Raster datasets have a large assortment of information beyond the basic pixel display. This information is stored in the properties and is helpful in understanding more about the data. Locating the properties of a raster dataset can be a tedious … Continue reading

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5 Minute Trouble Shooting – Changing the Units on a Profile Graph

“Why does my profile graph show an elevation change of 11 feet over 1,111,111 feet?”

In general, profiles show the change in elevation of a surface along a line. They help to assess the difficulty of a trail for hiking or biking, or to evaluate the feasibility of placing a rail line along a given route.

A Profile Graph represents height on the Y axis and horizontal distance on the X axis. The unit of distance along the X axis depends upon the units of the projected coordinate system (PCS) of the elevation raster data. For example, if data is in a Universal Transverse Mercator (UTM) PCS, the unit of distance will be in meters; if data is in State Plane PCS, the unit of distance will be in feet (US) as shown in the following figure.

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Esri GRIDs are always 32 bit

Bitdepth

There are many different kinds of rasters that can be used in ArcMap. To better describe and explain these different kinds  of rasters, Esri has created help documentation on technical specifications and supported raster formats.  Included in the documentation is … Continue reading

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Calculating the slope of a hiking trail

Add Surface Information MXD

Trail hiking can be a fun and exciting activity when you properly prepare for the hike.  An important step for preparation is knowing the trail.  Some key factors include: length, minimum elevation, maximum elevation, and slope.  If you do not … Continue reading

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Georeferencing in ArcGIS 10 or “Why is my georeferencing toolbar suddenly grayed out?”

Georeferencing Toolbar at ArcGIS 9.3.1 - After

Questions on georeferencing are some of the most common questions that we get in Support. These conversations range from basic how to questions all the way to questions like “Why are the buttons grayed out?”

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Using TINs Created with ArcGIS 10 in Previous Versions of ArcGIS

Have you opened your scene document to find a warning that the base surface for your 2D data cannot be found? The only thing different with this document is that it was saved as a previous version from ArcGIS 10.0 and you’re now opening it in ArcScene 9.3.1. Continue reading

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If Only This Topo Map Was More Interesting

Topographic maps are some of the most common maps that are in publication. They come in many shapes and sizes, but how can you really spice one up? What can you do to make a regular topographic map (like the one below) more interesting?

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Get Your Free Raster Data Here!

As a raster analyst, there are a lot times I try to “fix” many types of raster data from various online sources. If you’re going to get data from elsewhere, there are many reliable sources available for GIS data, some websites that charge a fee, and some that are really well-done free clearing houses and data centers.

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Null Doesn’t Mean Zero?

How to add different-sized rasters with NoData Values 

One of the most common forum topics is on how to add two rasters with different spatial extents and get a result more than just where the two rasters overlap. Well, the reason the result is only those areas has to do with null values.

There are environment variables to modify to ensure that the rasters line up (snap raster) and the extent is preserved during the process (Spatial Extent). Fixing the environment setting alone will still not fix the output of the process. If you modify the spatial extent to match the “union of inputs”, the output raster will have the correct spatial extent, but the only values will be the values where the two rasters overlapped. So what is going wrong?

Obviously it is the pixel values … or, wait, I guess it isn’t that obvious. Try this piece of wisdom provided by forum poster and Product Engineer, Eric R.

Pixel value + NoData Pixel = NoData Pixel
Pixel value + 0 = Pixel value
Pixel value + Nothing (because your extents are different) = Nothing.

Two different rasters used in calculation (NoData is red).

Sum of Rasters

You may be asking, “What is actually happening? Are the values being used in the analysis or not?” Well, the answer is they will be as long as part of that equation doesn’t include a null value. If there is a null value present that will make the output null, then the output will not be correct. 

The next question is how to fix it. Bear in mind, both solutions require you to make the rasters that are being used as the inputs match in spatial extent, pixel registration and cell size. You can use the Copy Raster tool and set the appropriate environment settings to accomplish this. Once you have corrected the rasters, then there are two options depending on the desired output,

1. Set the NoData values to a value of zero, so that when they’re added together, the output will have the correct spatial extent and values from both rasters.

Output from Setting the NoData value (blue pixels = 0)

2. Utilize the Con( IsNull ) workflow to include all of the pixels and add them where appropriate.

Con(IsNull(“RasterSMALL”),”RasterBIG, (“RasterSMALL” + “RasterBIG”))

Output from Con Tool

To recap, there are options to handle the NoData cells. Depending on what output you’re trying achieve, there are different techniques. It’s also important to understand the effect that NoData has when used in raster calculation. 

There is a related forum post that discusses using Python to sum multiple rasters together: Calculate sum of 2334 raster layers.

Jeff S. - Geodata Raster Support Analyst


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