This post comes courtesy of cartographer Jonah Adkins, from Newport News, Virginia. Jonah is a GIS alchemist, routinely crafting beautiful and insightful work, and sharing his process. … Vector tiles in Azimuthal Equal Area?? NO WAY! Way I’ve been learning … Continue reading
- Only updates your files- no redundancy created
- Update are almost immediate
- A new raster dataset is created and the original file is left unaltered
- Choose many options: such as file format, cell size, resampling technique, and NoData value.
- This is the only option for some files: such as GRID and other files that cannot be updated (e.g. read-only files)
Depending on your workflow, both Update Georeferencing and Rectify have their advantages.
Did you know that you can use the Viewer Window while you are georeferencing? This is especially helpful when you are trying to georeference your raster to another raster. The Viewer Window allows you to view the referenced and your unreferenced raster at the same time. Now you can zoom into the same location on the main display window and also on the Viewer window, so that you can choose your links without having to toggle the visibility.
You may want to turn Auto-Adjust off, so that the display does not move around while you are creating links. Once you are done choosing links, you can turn the Auto-Adjust back on or you can click Update Display to show you the georeferenceing result.
Written by: Simon Woo
There’s a lot of geographic data out there and most of it was created using a spatial reference. However, people are still dealing with issues of not knowing the spatial reference for some of their data, which includes imagery.
I generally see these four problems:
- What is a spatial reference?
- I know the spatial reference information but how do I apply it to the data?
- The spatial reference exists but the data appears in the wrong location.
- I don’t know the spatial reference for my data.
Spatial reference, projection, coordinate system?
If you’re asking “What is a spatial reference?” then you should start with some of these other options:
- Training: Understanding Map Projections and Coordinate Systems
– Mapping Center: Map Projection Animations
– Help topic: Georeferencing and coordinate systems
-Help topic: What are map projections?
– 2010 Developer Summit: Understanding & Using Geometry, Projections, and Spatial Reference Systems
– Book: Lining Up Data in ArcGIS: A Guide to Map Projections
Applying the spatial reference
Defining a spatial reference (projection) for your data is not the same as projecting your data. When you define the projection you’re simply identifying the coordinate system that is used to place you data’s coordinates in the correct location. For example, there are a number of raster dataset file formats that use an associated world file to define coordinate and pixel dimensions for the upper left corner of the raster dataset. Most commonly you’ll find .tif and .tfw files, or .sid and .sdw, or .gif and .gfw files. You need to use the Define Projection tool, otherwise the coordinate defined in the world file may place the image in the incorrect location. This tool will create an .aux.xml file that gets stored with your raster dataset containing the spatial reference information.
When you’re projecting data, you are changing the coordinate values and sometimes the shape of your image (due to the warping of the pixels to another location). You can change the projection of your raster data using the Project Raster tool. If your imagery has no spatial reference you may have to georeference it using the Georeferencing toolbar.
Finding the spatial reference
If you’ve opened up the data’s properties in ArcGIS and you’ve examined the data’s storage location for any other associated files, such as a metadata file, and you still don’t know the spatial reference, where do you start? Or if your image is displaying in the wrong location, what do you do? These problems are the same for vector data as for raster data.
Hopefully, you know where in the world your raster dataset is supposed to be. This usually helps because there are common spatial reference systems for each country, state, or province.
Next, look at the coordinates. You can open the world file (it’s just a text file), or add the raster dataset into an empty data frame in ArcMap and examine the coordinates as you move your mouse around, or look at the coordinates in the Properties dialog box. If the y-dimension is between between -180 and +180 and x-dimension is between -90 and +90, these are longitude and latitude values so your data is using a geographic coordinate system. If it’s recent data you can probably use the WGS 1984 projection (or at least start there). If these values are larger numbers, then it’s probably a projected system and these units normally represent meters or feet. In the USA you’ll typically find your data in a State Plane coordinate system (often using feet) or in a UTM zone (using meters).
Unknown spatial references are a common problem, but using the steps described in the following should help:
- Help topic: Identifying an unknown coordinate system
– 2010 UC presentation: Using Map Projections and Transformations
The above topics recommend various ways to help determine the correct system. One way I would recommend is to add a basemap in ArcMap (which typically use WGS 1984 Web Mercator Auxiliary Sphere projection), then modify the projection of the data frame until your image appears in the correct location. For example, start by changing it to the UTM zone projection in your local area, then change it to the State Plane zone. You can also choose a State Plane in feet or meters. You may also need to modify the datum. For example a datum using NAD83 may be a few 100 meters from one using NAD27.
See the above links for more tips. But if you don’t know the spatial reference then you need to become a geospatial detective to identify what’s correct for your data. If you can’t find the answer you may have to use the Georeferencing toolbar and reproject the data.
Submitted by: Melanie Harlow
by David Palomino
The UpdateMXDs tool updates Business Analyst 9.3 and 9.3.1 MXDs to Business Analyst 10 (it replaces the background layers). When updating a Business Analyst 9.3 or 9.3.1 map document (MXD), it is recommended that you enable Automatically update map projection on the Misc tab of the Business Analyst Preferences dialog box. (For more in-depth information, read this article.)
There are three ways to update your MXD from 9.3.1 to 10: from ArcCatalog, ArcMap, and ArcMap through the Catalog window.
1. Start ArcCatalog.
2. Add the UpdateMXDs toolbox (UpdateMXDs.tbx).
3. Right-click ArcToolbox and choose Add Toolbox.
4. Navigate to C:Program FilesArcGISDesktop10.0Business AnalystDatasetsUpdateMXDsUpdateMXDs.tbx and add the toolbox.
5. Open the tool and navigate to the folder that contains your 9.3.1 saved MXD(s).
6. Click OK.
When you navigate to where your MXD(s) are located, you’ll notice that there is a backup MXD for each.
7. Open the nonbackup MXD.
Due to Business Analyst 9.3.1 being run on geographic projection (projection on the fly) and 10 being run on Web Mercator, your zoom level will look different.
Note: Business Analyst 10 is in Web Mercator and is thus in line with industry standards as well as being consistent with ArcGIS Online map services. You can now more easily load cached background map services and layers seamlessly.
8. Start ArcMap and set the projection to Automatically update map projection.
9. You may still need to zoom in or out of your given geographic area. In this case, the map was zoomed in to the drive Ttme trade area.
1. Start ArcMap and set the projection to Automatically update map projection.
2. Open the UpdateMXDs tool dialog box and navigate to the folder that contains your MXD(s).
3. From ArcMap, navigate to your MXD and open it. Just as in the example in From ArcCatalog, you may need to zoom in or out to see your original extent.
From ArcMap through the Catalog Window
A third way of doing this is by simply accessing the Catalog window through ArcMap and following the steps outlined in the From ArcCatalog section above.
Recently we had an E-mail from an Explorer user claiming that MrSID images did not seem to be supported. “Not so!” we maintained, but yet this user had no luck in opening his MrSID images which were being downloaded from a Web site. We took a closer look, and discovered that the projection had not yet been defined for his MrSID files.
The short story is that all data sources to be added to Explorer must have their projection defined, otherwise Explorer does not know how to project the data on-the-fly onto its globe.
But here’s the long story, and how we solved the problem, so you can too if you run into it.
They key indicator that a projection (spatial reference) has not been defined for a data source is this message:
It’s easy to (wrongly) think that the .sdw file of the SID contains the projection. In fact, it only contains the georeferencing, and projection information is written to an .aux file once set.
To set the projection, we started ArcCatalog, navigated to the MrSID file, and right clicked the file to open its properties. You can see that the project has yet to be defined.
We clicked Edit, then navigated through the options to find the projection definition, going from Projected Coordinate Systems > State Plane > NAD 1983 (feet), then chose the file below for FIPS 3200.
Once the projection was defined, we could add it directly to ArcGIS Explorer.
The good news for Explorer users is that defining the projection for data sources is something normally done by the GIS analysts that author and manage the data. In this case it was missing, but is always good practice to define.
By Charlie Frye, Esri Chief Cartographer
Not long ago we received a question via Ask a Cartographer that our map projection experts frequently get; it goes something like this: “When projecting data to WGS84, for example, which transformation is the best to use?” Continue reading