- Do I know the difference between a geographic and a projected coordinate system?
- Do I know the difference between Cartesian, polar and cylindrical coordinate systems?
- What is an assumed coordinate system?
- When is it necessary to transform my data from one coordinate system to another?
- What is the appropriate transformation method to use?
- When I download data, how do I know what coordinate system it is in?
- How do know if my data are in the right place?
The data was collected using an assumed coordinate system on a cartesian grid. You will find it is very precise data with high relative accuracy, but its location in real world coordinates is not nailed down. Your goal is to apply a coordinate transformation of this data into projected real world coordinates (UTM Zone 12 N) so that the data can be overlaid with other geographically explicit data layers and datasets as illustrated in the map below (see task 3).
Bring the raw survey data into an empty ArcMap document, and create a map with a measured grid overlay to display the data, denote the different types of data, label the benchmarks, and illustrate that the data is unprojected in an assumed Cartesian coordinate system. While not spatially referenced, this data DOES have scale. It was collected in meters against the assumed grid.
You will notice that there are a number of data layers and some of them are empty. Please ONLY use the following layers for this task:
- Hard_Breaklines_Unprojected, and
When finished, include a static version of this map in your website. Save your map (maybe as Task 1) and quit ArcMap.
You may wish to wait to export this map until you have finished Task 2.
See here for more Task 1 detailed instructions.
The key to transforming any data from one coordinate system to another coordinate system is having some coordinate values for the same points (typically 2-5) in BOTH coordinate systems. In our case, we want to transform survey data in un-projected assumed Cartesian coordinate space to real world projected coordinates (NAD 83 UTM Zone 12 N), and we have three benchmarks to do it. The benchmarks were surveyed in with both a total station (very precise:
+/- 5 mm) in an assumed coordinate system, and in real world
coordinates with a very inaccurate hand held GPS (+/- 5 m). However, using the GPS coordinates as a basis for a simple affine transformation (i.e. a shift, rotate and datum adjustment), these coordinates will be good enough for GIS overlay purposes. The cartoon below illustrates conceptually the process of shifting and rotating the total station survey data using the GPS coordinates of the same points. In the example below, BM3 is used as the point around which the transformation is based (click on image for larger view). You will use the CHaMP Transformation Tool to help you.
CBW05583-028079 Benchmarks.csvand open these in Excel or Notepad.
- Install CHaMP Transformation Tool AddIn
- Download here;
- installation instructions here;
- Use Chrome not Internet Explorer;
- Install the correct AddIn for the version of ArcGIS you are using. If using ArcGIS 10.2 or 10.3, install the 10.1 AddIn).
See here for more detailed instructions.
Please share your transformed data with your audience (on web) in an interactive map format (Google Map like last week is fine, or if you want to try ArcGIS Online Explorer, that is fine too). See What to Submit below for submission expectations.
Remember, if you group your data together to make your kml file -be sure to remove the basemap. You can also use the "map to KML" tool, but again, remove the basemap, first. Save your data as layer files to preserve your symbology. And include a descriptive title, caption, and/or header for your interactive map to provide context for your web audience.
You can learn more about this particular site (CHaMP site CBW05583-028079) here.
- make two nearly identical maps of the data before and after transformation.
- Use a measured grid on each map,
- one displaying the assumed coordinate system,
- one displaying the projected coordinate system
- (each grid has a description of its coordinate system)
- Use the same symbology for each map so your audience focuses on the things that change, not on the bits that are the same.
- Describe how you evaluated for correctness by explaining visual checks, residual errors, and understanding of skew, scaling, translating, and rotating that might happen during the transformation.
- Brief introduction & Purpose of Exercise in your own words
- Task 1, show a figure of the unprojected data as described above.
- In a map caption include the coordinate system and the limitations of the data are in this form.
- For Tasks 2 & 3, create a figure to display the projected data
- In caption describe the coordinate system and how the transformation affects how the data can be used
- Report the residual errors associated with the transformation.
- Explain briefly why the affine transformation is a good transformation method for this data and project.
- Explain how you evaluated the success of the transformation.
- Choosing an Appropriate (Geographic) Coordinate Transformation - Its easy to get confused in ArcGIS, with so many types of transformations possible. The vast majority of the transformations available in ArcGIS are between two coordinate systems (geographic or projected), which are already defined. In this lab, you are transforming between an assumed local Cartesian coordinate system and a projected coordinate system.
- Spatial Adjustment (i.e. projected/unprojected coordinate transformations) - In ArcGIS, they bury transformation of vector datasets inside the editor toolbar, underneath More Editing Tools -> Spatial Adjustment. The spatial adjustment we did in this lab is known as an affine transformation (simple shift and rotate), and is one of several different transformation models.
- Georeferencing - The analogous process to spatial adjustment for transforming raster data from one coordinate system to another is known as georeferencing in ArcGIS (also commonly referred to as rubbersheeting or georectification). We will address georeferencing in Labs 9 and 10.
- CHaMP Transformation Tool - The tool you use in this lab to transform data from a local assumed Cartesian coordinate system to a projected real world coordinate system.
- Columbia Habitat Monitoring Protocol (CHaMP) - The monitoring program, which produced the data you downloaded in this lab and reporjected
- Lab 07 - Building DEMs - Taks 2 - If you're curious what can actually be done in terms of spatial analyses with the stream habitat survey data you transformed in this lab, you can peak ahead to Lab 7. In Lab 7 we take topographic stream habitat survey data, and construct digital elevation models from them. CHaMP is using these data to assess the status and trends of salmonid habitat conditions in tributaries throughout the Columbia River Basin.
- Wheaton JM, Garrard C, Volk C, Whitehead K and Bouwes N. 2012. A Simple, Interactive GIS Tool for Transforming Assumed Total Station Surveys to Real World Coordinates - The CHaMP Transformation Tool. Submitted to Computers & Geosciences.42: 28-36. DOI: 10.1016/j.cageo.2012.02.003.