Archive for the ‘Tools’ Category
Editing OpenStreetMap data with QGIS and Merkaartor
Problem: You want to contribute to OpenStreetMap from your desktop instead of from the Potlatch web tool.
In my last entry I discussed the process of downloading OpenStreetMap data using Quantum GIS (QGIS). This is how the downloaded data looked in the vicinity of the North Plateau Loop in Monte Sano State Park:
In this entry, I will cover two different methods I used to update this data to include my GPS track of the entire loop. I initially attempted to do this with QGIS.
Method #1: QGIS
When you first activate the QGIS OpenStreetMap plugin, the editing widget should appear on the right side of the screen. If it doesn’t, turn it on by going to View –> Panels –> OSM Feature. You should use the widget instead of the standard toolbar to identify and edit OSM features. If you click the OSM identify tool and hover over features, they will turn red and you can see their tags.
Shown above (in red) is one of the footway bits that fell near the North Plateau Loop, along with my GPS tracks (in magenta). I decided to keep the two segments that were already there, along with their foot=permissive and highway=footway tags. I added a name=North Plateau Loop tag to both of them by typing in the <new tag here> area.
Then I pressed the Create Line button.
The editor will automatically snap to existing features. I began at one the end of the segment by the parking lot and traced along the length of my track, stopping to incorporate the second segment along the way. Then I added the three tags to the new lines I had created.
Once I was happy with my edits, I tried to upload them to the OpenStreetMap server. At this point in the process, if you do not yet have an OSM account, go to https://www.openstreetmap.org/user/new and create one. Then, go back to QGIS and press the Upload OSM Data button.
The next screen is a summary of your changes. Add an optional comment to explain what you did, then enter your OSM account username and password, and press the Upload button.
If all goes well, you will get a message saying your upload has completed successfully. I, instead, got a python error message “OperationalError: SQL logic error or missing database”. According to the Quantum GIS forum, this has happened to some other people, but it doesn’t happen to everyone. You may get lucky. If not, you can try another tool such as Merkaartor.
Method #2: Merkaartor
While QGIS is a full GIS software package, Merkaartor has been developed solely for the purpose of working with OpenStreetMap. I found the interface less than intuitive.
After opening the program, my next step would typically be to add a basemap that moves me to the right geographic location. However, there is no Add Data button like I would expect. Instead, there is a list of layers already in the table of contents that have no source. You need to right-click on the top one to point it to some data. You can choose from a pre-loaded list of TMS (tile map service) or WMS (web map service) layers, or add a raster in GeoTIFF or “Walking Paper” (JPG, PNG or BMP) format.
I had trouble getting Merkaartor to load my georeferenced trail map, so I went the map service route. Once you pick a layer (I liked OSM Mapnik), you can use the mouse wheel to zoom and the right mouse button to pan to your area of interest. Then, press the green arrow Download button to get the current OSM layers in that area.
In the next screen that comes up, you would choose to download the Current View, which is selected by default. Alternatively, you could choose to download “From the map below” and do the same zooming and panning to your area of interest within the smaller window.
Merkaartor’s way of displaying the Monte Sano OSM layers is a bit different. It shows every vertex (node).
(click to see full size)
When you hover over a group of nodes, their color will change to a pink highlight. You must click on them to select them. They will then turn royal blue and their tags will be displayed in the Properties window on the bottom left of the screen. In order to select the line instead of the nodes that make it up, you need to move the mouse until the pink highlight turns thicker. After you click, the resulting royal blue selection symbol will also be thicker. Then you will see the tags shown above. You can edit them by typing inside the fields, just as with QGIS.
Go to File –> Import to load your GPS track .gpx file. It will be symbolized in a way that is difficult to distinguish from the OSM data. There is no way to change the symbols. I recommend turning off every layer that you don’t need to make things easier to see. (Do this by clicking on the eye symbol next to the layer.) When you are ready to begin editing, go to the Create menu and select Road.
I started, once again, at the bit of footway near the parking lot.
A couple cool things will happen at this point that didn’t happen in QGIS. First, the program will snap your new line not only to the ends of the existing segments, but also to the vertices of your GPS track. This makes it easier to follow it exactly. Second, the program will automatically carry all of the tags associated with the existing segment over to your new road. You will not have to go back and add them later.
When you have finished digitizing, press the ESC key to exit creation mode. At this point, if you want, you can polish up your work by splitting and merging segments. Simply hold down the CTRL key to select multiple segments, then choose the function to apply from the Road menu.
I found the ability to do that very useful because there was a bit of North Plateau Loop that didn’t belong in the original segments. Also, I worked in pieces so I could zoom in closer, and I liked being able to merge everything into one at the end.
When you are ready to upload your changes, you will need to go to the Tools –> Preferences –> Data tab to enter your OpenStreetMap username and password. Then press the megaphone Upload button. Review and summarize your edits in the next screen.
If you’re me, this time it will work! There is now a perfect North Plateau Loop on OpenStreetMap.org for all to see.
(click to see full size)
My parting thoughts are thus:
I wish I had been able to complete the upload in QGIS, because I find the program more pleasant to work with, and I like the idea of using a single GIS package to do everything. However, Merkaartor does offer some advanced OSM editing functions that make for a cleaner final product.
In summary, these are the main benefits of each tool:
QGIS
- Full-service GIS package
- Allows for changing of symbol representation
- Allows for raster georeferencing and display
- Better interface for those familiar with ArcGIS
Merkaartor
- Allows for merging and splitting road segments
- Snaps to GPS trace vertices
- Copies tags from attached segment into extension
- Uploads OSM edits without issues
Tags: free, gps, gpx, OpenStreetMap, Web Map Service
Working with GPS tracks in Quantum GIS (QGIS)
I am beginning to explore the world of free GIS software. QGIS has been recommended to me as one of the best out there. As an ArcGIS user, I found it easy to learn because the features are very similar. I am also impressed with the number of things it can do. I am going to write about just a small sample of them:
- Georeferencing raster imagery
- Loading OpenStreetMap data
- Viewing GPS tracks and waypoints (.gpx files)
My goal when I started this project was to contribute some new information to OpenStreetMap. Since I arrived in Huntsville, I have had a lot of fun walking around Monte Sano State Park. I also recently obtained a GPS unit that I’ve been using for car navigation, and I thought I could use it to record the path of one of the hiking trails.
On my first visit to the park, the gate attendant gave me a trail map that looked close to this.
(Click to see full size)
A blue line surrounding the parking area, called the North Plateau Loop, looked to me like a good place to start. It passes some lovely scenic viewpoints.
I wanted to check if this trail has been added to OpenStreetMap yet. I decided to georeference the trail map and then download the OpenStreetMap data that falls within its bounds. Georefrencing within QGIS is a little different than what I am used to because it uses a separate window.
To start the process, select Georefrencer from the Plugins menu. Then, press the Open Raster button in the new Georefrencer window, and navigate to your image. Mine was montesano.jpg.
Press the Add Point button to start adding control points. When you click on the image, a box will come up that allows you to enter x,y coordinates or load the coordinates from the map canvas (your other window).
I loaded some vector street lines into my other window that I got from the City of Huntsville GIS Department’s FTP site. I georeferenced to the street intersections. When you’ve added enough control points–at least three, but more is better, go to the Settings menu to open the Transformation Settings.
The QGIS User Guide explained that Thin Plate Spline is a good transformation type to use when you have a lower-quality original, because it will introduce local deformations. Cubic is a good resampling method for smoothing things like scanned maps where we don’t need to worry about maintaining precise grid cell values. The only output format available is GeoTIFF. Chose the save location, then press ok. To initiate the georeferencing process, press the green start arrow.
If you checked “Load in QGIS when done”, the image will show up in your map canvas table of contents, and you can check your work. Everything will stay loaded in the Georefrencer window the way you left it, so it’s easy to add or remove control points and try again.
Now, it is time to load OpenStreetMap data on top of my map. The OpenStreepMap plugin is included with the core QGIS software, but it isn’t turned on by default. Go to the Plugins menu and select Manage Plugins to check it on. This will add the OSM buttons to the toolbar and will open OSM widget to the right of the map window. (I will talk more about the widget in my next entry.)
I zoomed so the trail map filled the screen, then pressed the Download OSM data button.
QGIS will grab the current extent of your map window. If the area is too big, it will tell you and you will need to zoom in. Select a place to save your .osm file and then press the Download button.
QGIS will automatically symbolize the layers similar to how they look on the OpenStreetMap website. When I looked at the data, I could tell that someone had added a couple footways near the bottom half of the North Plateau Loop, but they were not connected or complete.
So, I headed back to Monte Sano to walk the trail with my GPS device. I have a Garmin Nuvi 255w. The OSM wiki has instructions for how to collect traces with this device, but I didn’t find that I needed to do all that. My Nuvi has a Trip Log feature that is actually always recordings paths. It saves them to a standard .gpx file. You don’t need to tell it to start doing that, but you can turn on the lines so you can see it working. To do that, press Tools on the opening screen, then Settings, and then choose Show under Trip Log.
Now, whenever you move, you’ll see a cyan line being built behind you. If you want to start anew without all the clutter from your previous trips, press Tools, then My Data, then Clear Trip Log. I told my Nuvi to navigate me to the front gate of Monte Sano. Then I turned off my car, but left the Nuvi on and took it with me. I walked the trail and watched the line grow.
When I got home, I plugged the Nuvi into my computer and grabbed Current.gpx from the Garmin/GPX folder.
I added it to QGIS along with the OSM layers I had downloaded previously. To add a GPS track, press the Add vector Layer button. You will need to select the GPX file type from the long list in the browse data box.
Then, a window will come up asking you to choose which GPX sublayers to load. Tracks is the only one I need for this project. Waypoints holds your saved favorite places, and track_points holds the time when each point along the track was recorded.
I changed the symbol of my track line to a thicker bright magenta so it would show up more clearly. You can get to the symbology settings by right-clicking on the layer and selecting Properties, just like you would in ArcMap.
You can see that the GPS recorded my driving, parking, and walking movements all in the same layer. I think that the wiki instructions I linked to above might have allowed me to isolate just the trail, but then I would have had to do an extra file conversion step, and it is pretty easy for me to tell which is which.
My plan next was to edit the OpenStreetMap data that I downloaded to include this trail, and then upload my changes to the OSM server. I will compare the process of doing that with QGIS and Merkaartor in my next entry.
Tags: free, georeference, gps, gpx, OpenStreetMap, trip log
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
Viewing flood zones with ArcGIS Explorer, Part 1
Problem: You need to look at some GIS data without purchasing a GIS software package.
This post is inspired by a reader question. My friend Jeremy asks,
“Is there any easy way to get at GIS data of water tables, flood zones, and stuff like that? Things that people who buy houses really should look into but seldom do.”
I cut my teeth making flood zone maps so I am quite familiar with where this data lives. I revisited the FEMA Map Service Center to give him some specific advice about where to look. My plan was to determine the easiest/fastest path to the information. To my dismay, the website has not changed much since I changed jobs. Meaning, to put it bluntly: there is no easy way.
I assume most visitors to the website would want to be able to type in an address and have that address come up as a dot on a map with the flood zone layer below it. If the dot falls within a flood zone, they’d be able to tell in one glance. Well, you can get pretty close, but you can’t exactly do that. So, instead of talking about the easiest way to get flood zone information, I’ll talk about four different ways.
I attended college at George Mason University so I decided to use the city of Fairfax as my example location. Pretend you are moving there. Because I don’t want to map a private residence, pretend you are moving there because you just got elected mayor. You want to check if your new digs at City Hall are in danger of flooding.
Method 1: FEMA’s Map Viewer
https://hazards.fema.gov/wps/portal/mapviewer
Pros
- This viewer allows you to type in an address and zoom to it.
- The map that comes up shows the flood zones in that area along with a lot of other useful information.
- There’s an identify tool that allows you access even more information about the data layers.
Here’s what I got for City Hall:
Cons
- There’s no “you are here” symbol on the map. If I’m not familiar enough with the area to already know where City Hall is, I still don’t know whether it’s in a flood zone.
- There’s too many layers turned on by default, which clutters the map, making it confusing and hard to interpret at first glance.
- The legend isn’t very helpful. It doesn’t explain what the blue-hatched Zone A is, or what the purple text LOMRs are. I only know that they are Letters of Map Revision because I used to work there.
Method 2: Product Search by Address…
Box on top-left corner of main Map Service Center screen
You can enter an address and select either a Flood Map (visual image) or DFIRM (digital database) product. The Fairfax City data is at the moment only available as a DFIRM, which I will discuss later in method 4. For now, I’ll enter the address of nearby GMU campus to bring up the Flood Map of surrounding Fairfax County.
Pros
- A beautiful looking map (If I don’t say so myself) which is simple and easy to interpret, and also happens to be the authoritative flood zone representation product.
- A good legend which explains in detail what the flood zone designations mean.
Cons
- Obviously no “you are here” symbol on this static map.
- And, it’s even harder to pinpoint an exact location than in the Map Viewer, because these maps only show major roads, and most of the time use route numbers instead of road names.
It isn’t hard to see why people may feel the need to look elsewhere to determine if their prospective property is really in a flood zone. Maybe that’s why a google search for “flood zone data” brings up companies that will research this for you if you pay them. There are, however, other ways, which are free or almost free.
Method 3: FEMA’s Web Map Service (WMS) & ArcGIS Explorer
https://hazards.fema.gov/femaportal/wps/portal/NFHLWMSe
A web map service is a computer program that will query a database and deliver the results back in the form of a dynamically generated map. FEMA makes the National Flood Hazard Layer available for free through a WMS. You need to have GIS software in order to read WMS data. However, if you don’t have access to the ArcGIS Desktop package that I regularly blog about, you aren’t locked out from playing. You just need to grab a copy of ArcGIS Explorer: “GIS for Everyone” at http://www.esri.com/software/arcgis/explorer/index.html
Once you’ve installed and opened the program, press the Add Content icon on the main toolbar. Select GIS Services… and then press New Server Connection. For the server type, chose WMS, and then enter the Server address that FEMA gives on their tool description page:
http://hazards.fema.gov/wmsconnector/wmsconnector/Servlet/NFHL?
You’ll then be given a list of services to choose from. Flood Hazard Zones (General) is the main one. If you pick that, then enter your address in the Find box on the left side of the screen, the globe will whirl around until you see this.
Eureka! Now I know City Hall isn’t in a flood zone!
(I changed the Basemap to Streets)
Stay tuned, in two weeks I will post about Method 4: Using DFIRM Shapefiles. The best is yet to come.
Tags: ArcGIS Explorer, flood zone, Web Map Service
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.
Randomly placing points with Hawth’s Tools
Problem: You need to position a specific number of points in random locations within a polygon
Randomization is sometimes needed for environmental sampling in the field. In my case I needed to produce maps of random soil sample locations within property boundaries. There is no easy way within ArcGIS to accomplish such a thing. Luckily, it is one of the many capabilities included with the fantastic Hawth’s Tools plugin. This plugin is available for free at http://www.spatialecology.com/htools/
Let’s put it to work in an ordinary neighborhood such as the one pictured below.
First, download Hawth’s Tools and run the setup package. Then, add it to your ArcMap toolbar by right clicking anywhere in the empty space, selecting Customize, and checking on Hawth’s Tools in the list of available toolbars. Before you run the tool, zoom in on your property and select it (indicated by the cyan outline). Once the parcel polygon is selected, chose Sampling Tools –> Generate Random Points from the HawthsTools drop-down menu.
A dialog box will come up in which you’ll need to specify:
- The Reference layer (layer within which you want to place the points). In this example that’s OwnerPLY, my parcel boundary layer. Check “Use selected features only” to limit the output to the single property you selected previously.
- Any layers where you don’t want points to show up. In this case, it isn’t feasible to take soil samples under the house, so I’ve told the tool to prevent points from occurring in BldgPly.
- Minimum distance between points, if you want that. I didn’t specify it here.
- The number of points to generate. Five in this example. Note: stratified sampling would create your specified number of random points in every parcel polygon.
- The name & location of your output shapefile.
The tool will generate five points following all those rules. It will add an ID column to the attribute table, filled with zeros. It’s an easy matter to change those zeros to numbered point IDs. Then, if you run your new shapefile through ArcToolbox’s Add XY Coordinates, and turn labels on for the ID field, you end up with this.
Give the field workers that map and table of coordinates, and they have what they need to go out and collect those samples. By the way, Hawth’s first Sampling tool: Create Random Selection, could be used to randomly select one of these 5 random points, for double duty.