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Using Earthmate® Blue Logger GPS and DeLorme Mapping Software to Create an Accurate Digital Map of a Parcel

Part II – Improving the Accuracy of a Digital Ortho Quarter Quad (DOQQ) and Finishing the Digital Map

Written by

Mark Genaris

Overview In Part I of this article, I discussed how I used the Earthmate Blue Logger GPS receiver and DeLorme mapping software to map the property boundaries of my newly acquired parcel of land. This was an important first step to my final goal of producing a comprehensive digital map of the property. In this article, I will discuss how I used Blue Logger GPS and XMap/GIS Editor to improve the accuracy of the aerial photograph (DOQQ) I downloaded using the NetLink tab in XMap. My goal was to create a more accurate base map on which I could not only overlay GPS data, but also digitize (trace) features such as buildings, timber lots, and wetlands, and get their square footage using the annotation tools in XMap.

Why Reregister Maps? DOQQ maps (a computer-generated image of an aerial photograph with the geometric qualities of a map) have an inherent accuracy of about 7 meters (approximately 23 feet). What this means is that a particular feature you see displayed on the aerial photograph (a telephone pole, for example) could be off by as much as 7 meters from where it really is on the ground. While 23 feet may be fine for some applications, I was interested in mapping the parcel data as accurately as possible. I discovered that DeLorme’s mapping software enabled me to create a localized DOQQ map with an accuracy of 2 meters or better. To accomplish this, I did what is known as reregistering, or shifting, a portion of the aerial photograph so that its features are more in line with the actual features on the ground. Once the aerial photograph is shifted, the GPS data displays more accurately. I can also use XMap’s annotation toolbar to digitize features from home and know that these features are within a couple of meters from where they really are on the ground.

Assigning Raw Data Logging Preferences Similar to Part I, I used Blue Logger Manager to configure the GPS receiver. The only difference in this case is that I configured the receiver to collect GPS points as raw data instead of standard data. As I discussed before, the standard data setting is primarily designed for situations where you are moving with the GPS unit while collecting data. However, since I was going to leave the GPS in one place this time, I chose to collect points as raw data. The benefit of collecting raw data is that I was able to use DeLorme’s GPS PostPro 2.0 software to post process the files, which greatly improved the accuracy of my points.

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Figure 1: Configuring the Blue Logger GPS to collect raw data.

Determining Remote Locations Before I revisited the parcel, I needed to find two remote locations (landmarks) on the DOQQ that I could locate on the property which had a clear view of the sky. In order to reregister the DOQQ, I needed to collect GPS points on each of these landmarks. These points will be used to shift the aerial photograph to improve its accuracy. I chose a tree stump as my first point and an obvious bend in the river as my second point.

Figure 2: The DOQQ showing the two locations I selected for GPS collection.

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Logging Raw Data GPS Points Collecting the GPS data was simple. I set up a tripod at each location and attached a Blue Logger GPS receiver to the top of each one. (I could have used one Blue Logger GPS and used a CORS site as my base location.) I didn’t have to set up multiple receivers, but wanted to determine the Setting up the GPS receivers in this way improved the accuracy of the data by reducing the number of satellite signals that reflected off the ground before they reached the receivers. I also made sure that I walked away from the receivers while they were collecting data (to reduce reflection off of my body). As before, collecting the data was achieved by simply turning on the receivers. I let each receiver collect data for 30 minutes to ensure more accurate results. After 30 minutes, I turned the receivers off and my fieldwork was completed.

Downloading the Raw Data Points Once I had collected the raw data, I connected the Blue Logger GPS device to my desktop computer using the wireless Bluetooth connection, and downloaded the GPS raw data.

Figure 3: Downloading GPS raw files to the desktop computer.

Post Processing the Raw Data Points for Greater Accuracy Since I wanted to improve the accuracy of my GPS data, I then opened GPS PostPro 2.0 to post process the raw files. The post-processing wizard made it easy to post-process the files and save them to my computer. In order to post process the data, I needed to have a remote location and a base location. A base station is a pre-determined position within 60 km of your current position (remote location). I could obtain base station information by creating a log at that location prior to creating the current GPS log file or by using available base station data (such as CORS—a network of base stations in the US maintained by the National Geodetic Survey, SOPAC—a network of international base stations, etc.).

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Figure 4: Post processing the GPS files.

Reregistering the Map with the Post-Processed Data Preparing the DOQQ for re-registration was actually a fairly simple operation. Since I was only interested in correcting a portion of the DOQQ, and since I didn’t want to keep any of its original coordinates, the first step was to “cut out” the portion of the map that contained the property and save it as an image file. The resulting file was simply a digital picture, minus any accompanying coordinates. To accomplish this, all I needed to do was display the DOQQ in XMap/GIS Editor, go to the Print Tab, ensure the print preview area encompassed the property boundaries (see below), and save it as a bitmap (.bmp) file using the save button (instead of printing the map). I then opened the bitmap file in an image editing program, removed the white border, and resaved the image. Now the image was ready to have new coordinates assigned to it so that it displayed in a more accurate location than the original DOQQ.

Figure 5: Preparing the DOQQ for reregistration. I now had all of the pieces I needed to reregister the digital image of the property: the cropped bitmap file of the property and two post-processed GPS points. The first step was to turn off the DOQQ and go to the Draw tab in XMap/GIS Editor. I then imported the two post-processed GPS points so they displayed on the map.

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Figure 6: Post processed GPS points imported to XMap using the Draw tab. The next step was to switch to the ImageReg tab in XMap and load the bitmap image of the property that I saved earlier. Once the image was loaded and visible on the left side of the screen, re-registration was a very simple process. The idea was to create two pairs of control points, with each pair corresponding to the same location on the map. In my case, I created the first pair to represent the tree stump and the second pair to represent the bend in the river. I placed the first point on the center of the tree stump on the left side of the screen, and then selected the corresponding GPS point on the right side of the screen as its match. I then placed the second point on the river bend on the left side of the screen, and selected the corresponding GPS point on the right side of the screen. With two pairs of control points established, I had ImageReg process the data and, in a few seconds, my reregistered DOQQ was ready to use. The new improved DOQQ automatically displayed on the screen, and I immediately saw that my GPS points were now displaying in the correct locations.

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Figure 7: Bitmap image of the property loaded into the ImageReg tab in XMap.

Figure 8: Creating two pairs of control points to reregister the bitmap image.

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Figure 9: Reregistered DOQQ displaying in XMap. Now that I had my reregistered image, I used XMap’s digitizing tools to trace some of the features on the map. Since XMap creates geo-located data about the objects you draw, I was also able to extract information about the features such as length and area. Because I reregistered the DOQQ, I was confident that these features were in the correct geographic location.

Conclusion When I combined the parcel boundaries from Part I with the data I created in Part II, my digital map was completed. I have been using it almost every day to obtain information about the property, and it has been an invaluable tool in the decision making process. I am continually updating the map with additional GPS data and features that I have drawn using XMap.

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Figure 10: Completed digital map of the property.