This page features troubleshooting questions students have asked for Intro to GIS Lab 3: Vector Geoprocessing
Q: Lab 3, Part 4, Step 5: I have successfully opened the Vt_USDA_Zipcode table, but when I hover over the button to add a field, it is unavailable. It says “the table or its schema is in read only” and I checked the properties and found that that is true, and under Metadata it says ” Show metadata from data source (read only) ” and I don’t know how to fix that?
A1: I honestly don’t know why you would receive any permissions related messages with the table. I’ve used this data in various labs throughout the years, and have never had that happen. Would like to try to figure that out though, because it really was not part of the lesson plan for you to grapple with permissions. You cannot edit the structure of a table that is not stored in a geodatabase. Once you import a table into a geodatabase you have full control over it. That said, if you make a change to the table structure (i.e. add or delete an attribute) you do need to save your changes. In the image below, note that you see two different save options. One in the top left corner (third icon from the left) and the other on the Fields Tab in the Changes Section. The latter of these is the Save button you need to “commit” to the changes you’ve made in your table structure.
A2: Curious about “incorrectly uploaded all my shapefiles”. There are two ways to get your shapefile into a geodatabase: 1) right-click the geodatabase and choose Import > Feature Class (or Feature Classes if you have more than one to import) and 2) right-click the shapefile and choose Export > Feature Class to Feature Class (or Feature Class(es) to Geodatabase). Either approach yields the same result. From there, you can add your data to a Project in several different ways, again, all ending with the same result
Q: I really didn’t understand how to tell the difference between length and shape_length or which one to use. At the end when there was only 1 record selected, I thought that couldn’t possibly be correct because the question stated “records” plural. I spent several long hours re-doing the lab with much frustration, only to realize too late I was right the first time. I could have used clearer instruction.
A: The Shape_Length attribute is a system field created by ArcGIS (just like OBJECTID and Shape_Area) when you import data into a geodatabase. A vector point geodatabase feature class only includes OBJECTID (the unique value assigned by ArcGIS to each feature in a dataset). A vector line geodatabase feature class includes OBJECTID and Shape_Length (the length of each line segment in the units of the data’s projection. A vector polygon geodatabase feature class includes OBJECTID, Shape_Length (which in this case is actually the perimeter of the polygon) and Shape_Area (the area of each polygon). Both Shape_Length and Shape_Area are measured in the units of the data’s projection. You cannot create, delete or update any of these fields. Unless there is some compelling reason not to use these measurements, Shape_Length and Shape_Area will always be the definitive measures because they are updated any time a change is made to the features in the dataset (e.g. via union or intersect). Sometimes, like when you’re working with land parcel data, there is a legal description which specifies the area of the parcel. This is a case where you defer to the legal definition of area instead of the Shape_Area attribute.
Q: Why were shape_length and length so different? is there a way to tell how an attribute value is calculated? sometimes I look at an attribute and the values assigned to it and I don’t understand the relationship or what the attribute is describing, espicially if the name of the attribute is vague
A: Occasionally you will find legacy attributes / measurements that were created with the original data, but never modified as the data were updated over time. That’s why the Shape_Length and Shape_Area fields are important to use. They are updated every time a geoprocessing operation is run. Otherwise you would read the metadata (the data documentation) which you’ll learn about in Module 6. For now you’ll just have to trust me on the interpretation of any attributes that you use (beyond the system fields identified earlier: OBJECTID, Shape_Length, Shape_Area.
Tips & Tricks: Things I discovered this week. If I press the ‘alt’ key and then click on a feature class under the Contents tab -> the map pane will zoom or pan to that feature. If I click within the map pane on a particular feature – and a little neon green dot appears and then disappears – a popup window will provide some essential information about that feature without having to look at the attribute table. One can also highlight and copy information from the popup window easily for later reference or to make it larger because I probably need reading glasses. I figured out that if I click to highlight a feature class within the Contents tab -> use ctrl + t -> the attribute table will appear for that feature class. Then I realized if I hover over some of the items in the menu after “right clicking” keyboard shortcuts are illuminated.
One tiny note the link below took a little massaging. When I tried to click over directly from the .pdf file the link was broken. Copy & paste the link into a text file indicated that there were some illegal spaces in the path.