Esri Technical Support Blog - Page 2

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?

One idea, make the map 3D and it’ll become … Ta Da! ... a topographic block diagram!

This topographic patch is a view based on three datasets (topographic raster, TIN, and multipatch) in ArcScene. The multipatch forms the bottom of the diagram and is based on the extruded result between the two TINS. The buffer polygon is used as the input feature class in the Extrude Between process. In practice, it’s good to have a 2D polygon that’s buffered just outside the edge of the elevation raster. Follow these steps to do the same to your own topographic map:

1. Create a TIN from the DEM. (You can also create a Terrain at ArcGIS 10.)
2. Use the Raster Domain tool to create a polygon of the elevation area.
3. Use Feature Class to Feature Class tool (disable the z value in the Environment setting).
4. Buffer the polygon to extend just outside the area (buffered 1 meter in the example).
5. Add Field and specify the base elevation. (This depends on the data; I selected a value of 1,200 with a bottom elevation of 1,310 for the elevation raster.)
6. Create a TIN from the new polygon and specify a hard line based on the new field.
7. Use the Extrude Between tool to create the multipatch.
8. Open ArcScene and add the multipatch and the raster that you’re looking to enhance.
9. Right-click the raster in the Table of Contents. Select properties > Base Heights and select the TIN you used.
10. Select the 3D Effects Toolbar, select the multipatch and then, set the priority of the multipatch to below the raster.

Now you will have a topographic block diagram. In the example, a topographic map was used, but the same workflow will work with a three-band raster. Keep in mind that with ArcScene, larger rasters and larger multipatches can cause issues with the display because ArcScene uses a lot of memory. Remember to check out the Optimizing ArcScene tips found on the ArcGIS Resource Center.Jeff S. - Geodata Raster Support Analyst

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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.

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

Con(IsNull("RasterSMALL"),"RasterBIG, ("RasterSMALL" + "RasterBIG"))

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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Many of the menus and processes, such as the new geoprocessing toolset for example, have changed in ArcGIS 10.0. One of the biggest changes in ArcGIS 10.0 is the switch from Image Server to Mosaic Datasets. 
Previously, in Image Server, you could use the Advanced menu to manually control the options when adding rasters to an image service.
However, there are several situations where rasters added to an ISDEF should not apply the internal overviews that are present with some raster formats. SIDs, and other rasters with multiple levels of pyramids, can affect the performance of image services. Since image services use overviews to increase performance, increasing the number of overviews will improve the performance. 
When SIDs are added to an image service with the Advanced process under the General Tab, there is an option to "Include internal overviews". If this option is unchecked the raster will be added without bringing in the pyramids that are in the raster. For highly compressed rasters like SIDs, there can be many levels of overviews built into the raster which affects the number of overviews created by the mosaic dataset or ISDEF. 
In order to replicate the same workflow in ArcGIS 10.0, the Add Rasters To Mosaic Dataset tool includes a section of Raster Pyramid Options. In order to prevent the internal pyramids from being added to the mosaic dataset, set the value to zero (0). This increases the number of overviews that will need to be created, but the performance should be improved. If this option is not used, the overviews that are inherent with the SID will be used in place of overviews. So, the choice from a design perspective is  whether to utilize the overviews of the rasters added to the mosaic dataset or to create new overviews for the mosaic dataset. This pyramid option is available for all raster formats, not just SIDs. 
 

To recap, the ability to apply or ignore the internal pyramids and pyramids of input rasters is available in the Raster Pyramid Options section of the Add Raster to Mosaic Dataset tool. Jeff S. - Geodata Raster Support Analyst

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Have you ever added a raster to an MXD and noticed that it looked different? Or created a mosaic dataset or raster catalog and suddenly, the rasters are so bright, you think, "What did ArcMap do to my raster?!"

Well, I'll save you a Tech support call. There is a relatively simple reason why your raster is now so bright: there is a "stretch" being applied to the raster, making it brighter.  Applying a standard deviation to a raster with statistics is the default behavior in ArcMap because the stretch will improve the display of most rasters, but there are a few situations where the stretch is not appropriate.  

In order to turn off the stretch, open up the properties menu of the raster from the table of contents and click Symbology. The dialogue may differ depending on the type of data, but the components are the same. To see if your raster has a stretch, look below the "Display NoData as" button to the section indicated as - you guessed it - Stretch. Under that, you may notice the usual default setting as Standard Deviation. To turn off the stretch, simply switch that setting to "None" and click Apply or OK. Your raster should now look more like the original if you are using a mosaic dataset or raster catalog. 

So, why did this happen and where was the setting to control this effect?  Well, it depends on the version of ArcGIS Desktop you're using. In ArcMap 9.3.1., the control will be under Tools > Option > Raster > General, titled "Display raster datasets with contrast stretching." Uncheck the box.

In 9.3.1:

In ArcMap 10, the dialogue will be under Customize > Options > Raster > Raster Layer, titled "Stretch Type". Select None. 

Both of the settings will prevent ArcMap from displaying the rasters with a stretch initially. By default, the stretch is automatically applied to all rasters with statistics added to ArcMap. Deleting the statistics will also prevent the display from being stretched. Related Articles

• KB Article on Sids stretching in an unmanaged raster catalog
• Improving the display of rasters
• Stretching Process in Image Service
• Stretching for Contrast

Jeff S. - Geodata Raster Support Analyst 

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