Posts Tagged ‘shapefile’
Viewing Flood Zones in ArcGIS Explorer, Part 2
This is a continuation of my last post about different ways to access flood zone data for the non-ArcGIS Desktop user.
Method 4: DFIRM Shapefiles
Digital Flood Rate Insurance Maps are available to download from FEMA for $10. They’ve offered a few free samples and Fairfax City happens to be one of them.
The data comes in several formats including shapefile. ArcGIS Explorer can read shapefiles. However, it will not let you add them to your map unless they have a defined projection.
The shapefiles in the Fairfax City DFIRM that I downloaded didn’t have their projections defined. I would assume this is the case with all of them. Luckily, they tell you the projection in the metadata. And luckily, projections can be defined with a file you can create using any text editor.
To find the projection, open the _metadata file in the Document folder. If you scroll down about 2/3 of the way you’ll find the Spatial_Reference_Information section. The most important parts are the Grid_Coordinate_System_Name, UTM_Zone_Number, and Horizontal_Datum_Name. The Fairfax City DFIRM is in Universal Transverse Mercator (UTM) Zone 18, NAD 1983 datum.
I used that information to have ArcGIS desktop create a projection definition file in the format used by all ESRI GIS software (including ArcGIS Explorer). It looks like this:
PROJCS["NAD_1983_UTM_Zone_18N",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",
SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-75.0],PARAMETER["Scale_Factor",0.9996],
PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
All you need to do is copy and paste that text into a text editor, remove any spaces, then save it as a .prj file. The name before the file extension should match the shapefile you are trying to use. The main DFIRM shapefile is S_Fld_Haz_Ar.shp, the flood hazard zone areas. So the projection definition file should be called S_Fld_Haz_Ar.prj
If you don’t want to copy and paste you can download it and put it the same folder with the shapefile.
FEMA uses UTM for all of its DFIRMs, but they do not always use the same datum. If you download a different one from Fairfax City, you will need to check the metadata for the UTM Zone Number and whether the datum is NAD 1983 or NAD 1927. If it’s NAD 1983, you can use the same text from above as a template to create your .prj file. Just change the two red areas to match what the metadata says:
PROJCS["NAD_1983_UTM_Zone_18N",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",
SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-75.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
If it’s NAD 1927, use this template and change the red areas
PROJCS["NAD_1927_UTM_Zone_17N",GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",
SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-81.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
(Remove any line breaks that I’ve entered for readability).
Now, Go to the Add Content button and select Shapefiles… Then browse to the ArcShapes folder and add S_Fld_Haz_Ar.shp. The shapefile starts out looking like this…
…which isn’t very helpful, but just wait. If you right click on the layer in the Contents window, you can change the symbol to something with edges. Now you will be able to see the flood zone borders.
And, if you right click on the layer again and this time bring up the Properties window, you will be able to select certain attributes to show as Popup Content.
(click on the image to see full size)
When you open this dialog box, a list of all the available attributes will come up. You will be able to select which ones you want to appear in a little pop-up window whenever you click on a feature. I picked all of them. Then, in the bottom half of the box you can select one attribute that will appear whenever you hover over a feature with your mouse. I picked FLD_ZONE because this is the most important piece of information. Now, if I type the address of City Hall into the Find box and press enter, I have everything I wanted at the beginning.
There’s a “you are here” symbol, and if I mouse-over I see it’s located in Zone X (not Flood Zone). I can mouse over other areas to see where the nearest 0.2 pct annual chance flood hazard zone is. And if I click in the zone, I can get any more information that exists about it.
Knowing how to use shapefiles in ArcGIS Explorer opens up a world of information. You can watch a free basic overview of the software at http://blogs.esri.com/Support/blogs/esritrainingmatters/archive/2009/12/03/explore-arcgis-explorer-in-a-free-training-seminar.aspx
Tags: ArcGIS Explorer, attribute table, coordinate system, DFIRM, flood zone, shapefile
Using ET GeoWizards to enhance shapefile management
This week I am writing about yet another plugin that has proven quite useful to me. I have only scratched the surface of its capabilities, but the few features I did try out, I found reasons to use over and over. The plugin is ET GeoWizards, developed by Ianko Tchoukanski, and available at http://www.ian-ko.com/
This tool duplicates some of the capabilities already present in ArcGIS, however it makes them all available at the ArcView license level. This can be a real boon to people who don’t have ArcEditor or ArcInfo licenses. In addition, you might just prefer the way it handles things better.
For example, my favorite function is the most basic: “Create New Shapefile.” I love it because it lets me create new shapefiles right in ArcMap! No need to disrupt my workflow to start up ArcCatalog and then drag the file into ArcMap so I can start editing it. When you initiate shapefile creation, it lets you chose a spatial reference based on your current map or other layers.
Then, it lets you add attribute fields! This too would be a separate step doing it the ArcCatalog way.
Then, it dumps your new shapefile right into the Table of Contents, ready for you to start using it. This is so much more convenient that you will never want to go back to the old way. It alone is reason to get the plugin, but there is more.
My second favorite function is “Redefine Fields,” which lets you change the length of string fields, or the precision of number fields, in your attribute table. I have run into many cases where I needed my text fields to be longer, and this is the easy answer. The only way to increase field length otherwise is to delete the field and re-add it with different definitions. This is problematic if the field is already populated with data. You end up having to create “holder cells” and migrate the data back in.
This tool removes the need for all those intermediate steps. However, it does save the results into a new shapefile, instead of updating your existing shapefile. That isn’t ideal, but I think there’s no way around it. It is still a much simpler solution overall.
My third favorite function is “Order Fields” which changes the order of the fields in your attribute table. There is no other way to do this. Sometimes, you want the most important information to be in front, especially if you have lots of fields to sort through. With ArcMap, you can drag fields to reorder them, but they snap back to their original position once you close the attribute table. This tool changes their order for good. Also, if you want to remove any fields while you’re at it, you can do so by leaving them over on the left side. Like “Redefine Fields”, it saves the results into a new shapefile.
A couple other functions that I haven’t tried, but which look really good:
- “Generalize,” which reduces the number of vertices used to represent a polyline or polygon. Sometimes you will end up with a feature you need to change the shape of that has vertices packed so tightly you are going to be there all day dragging things. Problem solved!
- “Shape to ShapeZ” conversion, which will add the Z dimension to a shapefile. Z values allow for the storage of elevation data. You can’t load Z-enabled data into non Z-enabled feature classes. You have to drill down into the Environment Settings in order to enable Z values. It’s tricky enough that I will probably blog about it at some point. This looks to be an easier way.
As I said above, I have only begun to explore the free ET GeoWizards functions. The registered version has even more capabilities. And, there are ET GeoTools for inline editing and ET Surface tools for working with raster elevation data. Plenty of reasons to see what’s at http://www.ian-ko.com
Tags: attribute table, shapefile
Using shpchk to fix damaged shapefiles
While I was working on this week’s blog entry, I opened an old MXD, and noticed that one of the layers in the table of contents had “gone bad.” The checkbox had grayed out and there was a red exclamation point beside it. This is a normal occurrence when a dataset gets renamed or moved to a different location, and can be repaired by updating the link. But in this case, I hadn’t been moving any files around. I attempted to “refresh” the link, by re-pointing it to the same data source, but was stopped in my tracks by this error message.
Detour! I know a way to solve this problem, so this week, instead, I will be blogging about shapechk.exe
Problem: You get a cryptic error message when trying to load a shapefile.
Shapefiles sometimes get corrupted. The errors they produce as a result used to be even less descriptive than what you see above. But as that message states, it has something to do with there being more or less entries in the attribute table (dbf) than there are in the index of spatial geometries (shp). I don’t know how it happens, but I can tell you that it does happen often enough that you will encounter it eventually.
A long time ago, Andrew Williamson wrote a utility that he doesn’t support anymore, but still offers on the web at http://www.geocities.com/SiliconValley/Haven/2295/howto_shapechk.html Note: GeoCities is closing on Oct 26th (tomorrow). After that, you can get it at ArcScripts.
It still works. It may not solve all your shapefile problems, but, up to now, it has solved all of mine. It’s a standalone executable that you simply unzip and run. Then, you press the buttons starting with Shapefile… on the left.
Browse to your damaged shapefile, then press Build Shx. Shx is the shapefile index. It will generate a new one in case your old one was the problem. Next, Check DBF.
This is the part where where it checks if the number of records in the dbf matches the number of records in the shp. If you lost your dbf entirely, it will build you a new empty one. You won’t have attributes but at least you will be able to see your shapes. In my case, the utility needed to insert 28 empty records into my dbf table to fix my shapefile.
When I opened my MXD back up, everything was working again.