Attributes Form Configuration

• Default values
  • Examples of useful default values
  • Open local files using default values
• Constraints
• Drill-down forms

WARNING

Our mobile app was redesigned. We are in the process of updating this content to reflect these changes.

In addition to setting up edit widgets and designing form layout, extra configuration can be done to the fields in QGIS to make the data collection easier and more consistent.

Default values

Default values can be used to automatically record, e.g, the name of the surveyor, date and time of the survey, latitude and longitude of the feature, or to have frequently used values filled in advance.

TIP

Clone documentation/form_setup (opens new window) to follow this example!

The default value can be a text, number or a QGIS expression. Here we are going to set the default values for x and y field to be longitude and latitude.

1. Right-click on survey layer and select Properties
2. In the Attributes form tab, select the x field in the Available Widgets column on the left
3. In the Widget Display, set the Alias to Longitude
4. In Defaults tab, define the Default value as:
   x( transform( $geometry, 'EPSG:3857', 'EPSG:4326'))
   Note that the expression transforms the point from EPSG:3857 (map and layer coordinate reference system) to EPSG:4326.
5. Check the Apply default value on update

Repeat the same steps for y field and setting the Alias to Latitude and the default value to
y( transform( $geometry, 'EPSG:3857', 'EPSG:4326'))

Don't forget to save and synchronise your project!

When you survey a new point in Mergin Maps mobile app, you will see the values are automatically filled in:

Examples of useful default values

There are some commonly used default values that can be useful in your field survey. As they are filled in automatically, they can be hidden from attributes form.

• It is convenient to know when a feature was created and when it was last updated. Use fields with Date or Date&Time data types with the now() function to record these information. You can change the formatting using Date/Time widget.
• Similarly, the name of the Mergin Maps user who created or modified the feature can be recorded using the @mergin_username QGIS plugin variable. These field should have the Text (string) data type.
• The coordinates of a point feature can be recorded as well using the $x and $y function in QGIS. To record the coordinate accurately, these fields should have the Decimal number (real) data type. If the coordinates are in meters, values can be rounded to, say, 2-3 decimal places. When working with geographic coordinates that use degrees, you may want to round the coordinates to 8 decimal places. Use the apply default value on update option so that you have correct values when the position of the point feature changes.

Variable name	Sample value	Apply default value on update	Description
@now	2024-06-30 10:00:00	no	The timestamp of when the feature was created.
@now	2024-06-30 10:30:00	yes	The timestamp of when the feature was last updated.
@mergin_username	sarah	no	Name of the user who created this feature.
@mergin_username	jack	yes	Name of the user who updated this feature last.
round($x,2)	1898789.92	yes	The X coordinate of a point feature, rounded to 2 decimal places.
round($y,2)	6134520.89	yes	The Y coordinate of a point feature, rounded to 2 decimal places.

Open local files using default values

Default values can also be used to open local files (e.g. a PDF file) from within the form. This file needs to be packaged with the project, so it should be stored somewhere in the project folder.

There is a public project documentation/forms-display-images-and-files (opens new window) you can download or clone to see how the setup works.

• A PDF file named my-pdf.pdf is stored in the main project folder.
• The survey layer has a field named local-file-default-value with Text (string) data type.
  • This field is set to not editable (the form will refer to the same file for all features and does not need to be changed).
  • This field uses the Text Edit widget with the Multiline and HTML options enabled.
  • The default value is set to:

  '<a href="project://my-pdf.pdf">Open File</a>'
  

In the mobile app, you can tap the Open File link to open the PDF file using the default application of your device.

Open local files using the HTML widget

Local files can be displayed in the form also using the HTML widget.

In the documentation/forms-display-images-and-files (opens new window) project, you can explore and compare both alternatives.

Constraints

When collecting data, you may want to apply constraints to certain field(s) to avoid mistakes when the values are filled in in the field.

TIP

Clone documentation/form_setup (opens new window) to follow this example!

Here, we will set up a constraint to the number field that represents the number of plants.

1. Right-click on survey layer and select Properties
2. In the Attributes form tab, select the number field in the Available Widgets column on the left
3. In the Widget Display, define the Constraints:
   • check the Not null and Enforce not null constraints
   • Expression type "number" >= 1 and check the Enforce expression constraint option

When surveying new features in Mergin Maps mobile app, this field will have to be filled in using a value higher than or equal to 1. Otherwise, you will get a warning.

Drill-down forms

Cascade or drill-down forms enable to list values in a field depending on a value selected in another field.

TIP

Clone documentation/form_cascade (opens new window) to follow this example!

In this project, we will work with the point layer named car. The survey tab in the form contains information about cars, such as Car-Type, Car-Manufacturer, or Car-Model. These attributes refer to non-spatial tables using value relations.

To make the survey easier, we will set the drill-down form so that the values in the drop-down menus are filtered based on the previously selected values. For example, if Car type (truck/car): Trucks is selected, the Car-Manufacturer field will offer only truck manufacturers DAF Trucks, Scania, MAN. Likewise, the next drop-down menu for Car-Model will change accordingly based on the Car-Manufacturer selection, showing only relevant car models.

To set the drill-down form:

1. Right-click on the car layer and select Properties.

2. Navigate to the Attributes form tab. We will set the value relations for the Car-Type, Car-manufacturer, Car-model fields, which are located in the Survey tab of the form.

3. The Car-Type field refers to the car-type table. The key column is the fid and the value column is the name field:
   

   The car-type table is structured as follows:
   

   When entering data into this field, the user can choose from these four name values.
   

4. The Car-manufacturer field refers to the car-manufacturer table. This table contains the fields fid, name, and type.
   

   • Set the fid field as the key column and name as the value column.
   • Use the type field to define which values will be shown in the form based on the selected Car-type by entering this Filter expression:
     "type" = current_value('Car-Type')

Now, the form will offer only options, where the Type field of the car-manufacturer table matches the current value of the Car-type. When entering data, we will get a drill-down form: since Truck is selected as the Car type, only three manufacturers are available in the Car-Manufacturer field.

5. Car-model works similarly. It refers to the car-model table, limiting options based on the Car-manufacturer field.
   • Set the fid field as the key column and name as the value column.
   • Use the manufacturer field to filter the values using this Filter expression:
     "manufacturer" = current_value('Car-Manufacturer')

When doing the survey Mergin Maps mobile app, you will see that after selecting Car-type: Car, the Manufacturer field will only offer Mercedes-Benz, Skoda and Audi. After selecting Audi, the Car-Model field will only offer the appropriate car models.