Make ArcView Shapefiles from the Feature-Types6
The primary difference is that, instead of getting just one shapefile (posnpnt.shp or posnline.shp) you will get several:
• Not In Feature data will be in POSNLINE.SHP.
• Streets will be in STREETS.SHP.
• Rocks will not be represented as a shapefile since we didn’t collect data for this feature.
• Trees will be in TREES.SHP.
• Intersections will be in INTERSEC.SHP, since Pathfinder Office only outputs the first eight characters of a data dictionary feature-type as an ArcView shapefile name.
• Hydrants will be in HYDRANTS.SHP
• Streetlights will be in STREETLI.SHP
{__ } Start with the Export Utility. For the input file use F121723C.COR, which is in __:\GPS2GIS\Feature_Demo (pointed to by the Project named Feature_Demo).
{__ } The Output Folder should automatically come up as _:\GPS2GIS\Feature_Demo\EXPORT. Fix it if it doesn’t.
{_} For practice make a New Export Setup: Press the button "New Setup." From the drop-down list select Arc View Shapefile. The Setup Name will appear as "New ArcView Shapefile." Change it to "AV#4_yis." A window, Export Properties, will appear.
Under the Data tab: for "Types of Data to Export" pick "Export All Features" from the drop-down list.
For Output: Combine all input files and output them to the project export folder.
Under Attributes: Output the Time Recorded for All Feature-Types. For Point Features use the Height and the Point ID. For Line Features select the 2-D Length, the 3-D Length and the Line ID.
Units: You can use the current display units, since those let you see both the GPS track and features as well as the DOQ image. (You would have to be careful using the setup AV#4_yis in the future, since it would use the display units in effect at that time.)
Position Filter: make sure Differential is checked under "Filter by GPS Position Info."
Coordinate System: again, you may use the current display coordinate system.
The ArcView Shapefile tab, which allows you to Export Tracking Themes, needs no attention here.
ESRI has produced an ArcView extension called the Tracking Analyst that allows real-time tracking of GPS receivers, or "playing back" data taken by such receivers. There is some additional discussion of this ability in the next topic. Basically, the Tracking Analyst allows the user to plot GPS data as it is being acquired, by virtue of radio transmissions, perhaps by communication satellite or cellular telephone, of the GPS data to a location that then relays these data to the ArcView software. If, for example a bus company wanted to know the whereabouts of each bus and whether it was on schedule, it might use the Tracking Analyst to display this information in real-time. Click OK to exit the Export Setup Properties window. If you need to make a change or to be sure you’ve made the right choices, you can select "Properties" in the Export window.
{__ } When you are satisfied with the Export window, click OK. Pathfinder Office will commence making the shapefiles.
{__ } Click the folder icon on the Project Toolbar to bring up the project folder. Double-click the Export folder icon. You will see three files for each feature-type. For example, HYDRANTS.SHP, HYDRANTS.DBF, and HYDRANTS.SHX. HYDRANTS.DBF is a database file that contains the attribute information about the hydrants. This information will appear in the ArcView table of HYDRANTS.SHP. HYDRANTS.SHX is a file that ArcView uses for its own purposes.
Now you may:
(a) examine your own GPS data file and turn it into ArcView shapefiles (use your Project named FEATURES_yis, that references the directory:
_:\GPS2GIS\DATA_yis\FEATURES)
OR7
(b) proceed to examine the shapefiles you just made in ArcView in PROJECT 7D.
PROJECT 7D
Viewing Shapefiles
Here’s a summary, mainly for those who didn’t do the previous two projects: In PROJECT 7B we completed a data dictionary, installed it in the GeoExplorer, collected data under the structure of that dictionary, and brought those data back to the PC. In PROJECT 7C we looked at some similar sample data, and generated ESRI Arc View shapefiles.
From this point forward in the text, anyone with access to Arc View may process the files that resulted from the previous work.
{__ } Start ArcView. Make its window occupy the entire screen. Make a new View. Make its window tall, occupying half the screen.
{__ } Add the theme __:\GPS2GIS\Feature_Demo\EXPORT \STREETS .SHP (it’s a Feature Data Source) to the view and turn it on. Make sure it is active.
{__ } Open the theme table for STREETS.SHP. For each streets feature that was collected you should see a record giving both the features you specified when the data were taken (width and pavement) plus the features that were specified when you generated, with the export setup, the shapefiles. Any numeric units will, of course, be those you indicated at the time of export. Dismiss the table.
{__} Add the Image Data Source FIVE.TIF from __:\GPS2GIS\ Feature_Demo as a theme. Drag it to the bottom of the table of contents.
{__ } Make the View active again and add the feature-based theme INTERSEC.SHP. Turn it on. Make the cursor active with the Identify icon. Click on each intersection and verify that the T-Intersection designation (Yes or No) is correct. See Figure 7—5.
{__ } Open the other shapefiles generated in the preceding project. Look at their theme tables.
{_} Display the "Bulb_type" for the streetlights on the map: Make the theme STREETLI.SHP active. Select Theme ~ Properties ~ Label Field ~ Bulb_type. Click the Label icon (looks like a paper gift tag with a string). Click each streetlight feature.
{_} If you have ArcInfo and wish to convert each shapefile into a coverage you may use the ArcInfo SHAPEARC command.
Now Use Your Data
{__ } If you collected data in PROJECT 7B, go through the procedures of PROJECTS 7C and 7D, using your own data, using appropriate datum, coordinate system, projection, and units. Use the folder __:\GPS2GIS\DATA_yis\FEATURES that is specified by the Project you made in PROJECT 7B.
OVERVIEW
Planning a GPS Data Collection Session
Fact: In a given area, the GPS data collection process works better at some times than at others. Determining a good (or just satisfactory) time to take data is called "mission planning."
Several years ago, this discussion of mission planning would have had to appear at the front of the topic, because there were so few GPS satellites in orbit that you had to go into the field at specified times to collect data. Now with the NAVSTAR system at full operational capability plus some 28 satellites up and healthy at the turn of the century you can almost always "see" enough satellites to get a pretty good fix. But is it a fix that is accurate enough to meet your needs? (Our needs in this topic were simply to show you how the system worked. So we left the mission planning details until now.)
Figure 7-5. After Export, using ArcView to view features.
It might have been simpler if the GPS satellites were simply parked over various areas of the world so that, in a given location, you always dealt with the same set of satellites in the same positions. Unfortunately, this would mean that they were always parked directly over the equator, since, due to the laws of physics, that’s where all geo-stationary satellites are. (If they were orbiting the Earth anywhere else, as they would have to be in order to provide worldwide coverage, they would be moving with respect to the ground underneath.)
So the rule for GPS satellites is "constant change." The number of SVs your receiver can track changes. The geometric pattern they make in the sky changes. The local environment in which you want to take data changes— perhaps there are blocking signals from reaching your antenna. If you want to collect data at the optimum times—for examples, when the DOP is very low, when there are enough satellites in the right part of the sky, or when there are a large number of satellites available for overdetermined position finding—you might want to use mission planning software.
This software will help you if your data collection effort might be impeded because there are barriers between you and some part of the sky. You may use the mission planning software to simulate such barriers and to tell you when the satellite configuration will be such that you can take good data anyway.
Almanacs
A GPS almanac is a file that gives the approximate location of each satellite. Actual position finding does not depend on the almanac; a much more precise description of each satellite’s position is needed for a GPS fix calculation. (Precise descriptions of the orbit of a given satellite come from an ephemeris message—a broadcast that may change hourly—by the satellite itself.) Rather, the almanac is part of the general message which comes from each satellite, describing the orbits of all satellites. The almanac gives parameters from which the approximate position of all satellites can be calculated at any time "t" in the future.
Since it doesn’t have to be very precise, an almanac may be good for months: left to themselves, GPS satellites deviate little from their projected orbits. However, the DoD may move a satellite forward or back within its orbit—perhaps to replace a failing satellite or to make room for the launch of a new one. Activities such as this make it necessary to collect almanac information frequently. In fact, the GeoExplorer collects an almanac every time it is outside and tracking one or more satellites for a minimum of about 15 minutes.
This almanac can be transferred to your PC. While the receiver will not tell you the almanac’s age, the Quick Plan software (which is part of Path-finder Office) will tell you if an almanac is more than a month or so old.8 An ephemeris file, from which Quick Plan may derive an almanac, may also be loaded onto a PC across the Internet from the Trimble Navigation web site. Almanacs are used for at least two purposes:
• giving information to the receiver so that it can locate the satellites more quickly, and
• providing information to PC software so it can tell the user the best times to collect data.
It is this second purpose we examine here: using Pathfinder Office to tell us when we can get the best data at a given location.
Using Mission Planning Software
Trimble Navigation provides software to help the GPS user determine the best time to collect data: a program running Microsoft Windows called Quick Plan. Basically, Quick Plan gives you information about the expected behavior of the satellites. This is information you need for planning important GPS data collection sessions. To practice using the mission-planning software, do PROJECT 7E. To see the effects of poor mission planning, do PROJECT 7F.
STEP-BY-STEP
Planning a GPS Data Collection Session
PROJECT 7E
Setting Up for the Planning Session
The first chore that has to be done is to get current information as to where the satellites are into the Quick Plan software; there are at least two ways to do this.
1. You can go to a Trimble Navigation web page, which, at the time of this writing, is www.trimble.com/support/files/index.htm and download a file named current.ssf. You may put this file anywhere you want, but Pathfinder Office and Quick Plan use C:\Program Files\Common Files\Trimble\Almanacs as the default directory. This download process transfers an ephemeris file. An ephemeris file contains precise orbital information for each satellite; Quick Plan can derive an almanac from it.
2. You can transfer the almanac from the GeoExplorer. An almanac contains rough orbital parameters for every satellite in the constellation, and is broadcast, for all satellites, by each satellite. An almanac file is about one-seventh the size of an ephemeris file.
{_} Prepare to get satellite position information into your PC: Plan to either transfer it from the Trimble Navigation web site, or get it from your GeoExplorer. (If you did Exercise 2—1 you have already transferred an almanac.) A complete almanac file is recorded each time the GeoExplorer is exposed to at least one satellite for more than 15 minutes. To check your unit for a complete file:
• turn on the GeoExplorer,
• let it complete its startup routine,
• get to the main menu,
• press and hold the on-off key, and
• observe the bottom line of the screen; you are looking for a capital "A," which indicates that the unit indeed has a complete almanac stored in its memory.
If, instead, you see a lowercase "a," the unit does not have a complete almanac and you will have to either take it outside to collect data for approximately 15 minutes, or find another source for an almanac. Of course, the almanac in your receiver may be old. When was the last time this receiver was outside?
{__ } Assuming you are going to transfer the almanac, connect the GeoExplorer to the PC as though you were going to transfer position data files. Turn it on.
{__ } Start up the Pathfinder Office software. Under the Utilities Menu pick Data Transfer (or use the Data Transfer icon on the Utilities toolbar). The Data Transfer window will appear. Verify that "Status" shows that the PC is connected to the GeoExplorer. For "Data Type" select "Almanac." If you want to change the destination directory to something different from C:\Program Files\Common Files\Trimble\Almanacs now is the time to do it.
{_} Press the "Transfer" button. You are given the option of providing a new name other than "almanac. ssf." If you want a different name type it in. A name indicating the date the almanac was taken might be useful. Okay the box. Click "Yes" to overwrite a previous file by the same name if you want the latest almanac to register. The transfer should take place, indicated by a progress bar.
{_} Click "Disconnect." Dismiss the Data Transfer window.
{__ } Turn off and then physically disconnect the GeoExplorer from the PC. (And, disconnect the GeoExplorer from the power supply— at the large end of the cable.)
