Field surveying is one of the core activities involved in the production of geospatial information. It involves the use of hardware, software and data collection techniques to collect information that is organised based on spatial relationships. Producing a successful field survey requires adequate planning and consideration of factors such as the technology that will be used, who will be conducting the survey and how the collected data will be used. In this article, we will discuss how to plan a field survey using QGIS and Mergin Maps.

About QGIS

QGIS is a desktop GIS platform that allows users to visualise, edit and analyse geospatial data. It is published using the open source GNU GPLv2 license. The software can be downloaded for free, though a donation is recommended to help fund the continued maintenance and development of the software. Due to its accessibility and vibrant open source community, QGIS is one of the most popular GIS software platforms in use. It is capable of reading and editing the majority of vector and raster data formats used by GIS professionals such as GeoPackage, Shapefile, PostGIS and CSV.

About Mergin Maps

Mergin Maps is a field surveying application based on the open-source QGIS platform which is designed to make field surveying accessible to anyone with a mobile phone. Users can design a project in QGIS and deploy it to mobile Android and iOS devices where data can be collected and synchronised with the QGIS project. It also supports collaborative working with project versioning and various levels of user access. Mergin Maps is also designed with a user-friendly interface that enables users with limited technical experience to easily use the application to collect data.

Before you start your data collection project

What data are you going to collect?

The first step in planning a field survey is to consider what data you will need to collect. This will determine the types of layers and data formats you will need to include in your field survey. Field surveys are typically collected as vector data in one of three main formats including points, lines and polygons. In order to determine which of these you will use, you will need to consider what objects you will be recording in the field and how you want them represented. 

Points

Point data represents a feature with a set of spatial coordinates. It is useful for showing the location of distinct objects such as a tree, building or access point. Point data is non-dimensional meaning that it has no length or area. 

Lines

Lines are a series of points linked together to create a one-dimensional feature. Lines are commonly used to represent roads, paths, boundaries, power lines or other features that need to have a defined length. 

Polygons

A polygon is a series of lines that link together to create a closed, two-dimensional shape. Polygons are often used to represent property boundaries, building footprints, special zones or other features that represent a defined area on a map. 

When choosing which data type to use for particular features you will be recording in your survey, it is important to think about the level of detail necessary to represent the spatial relations accurately without including excessive detail that may become confusing or use too much storage. For example, if you are conducting a census survey of 1000s of addresses in a city, it may be sufficient to represent each address with point data. On the other hand, if you are creating a survey of water utilities, it may be necessary to represent building footprints and property boundaries with polygon data so you analyse the spatial dimensions of the areas in more detail. 

How to choose data types for your survey project

After choosing the spatial data types for your project, you will need to consider the types of data that will be collected in the survey forms of your project. Each field in the survey must have a data type. Selecting different data types will determine what information formats can be collected. You can have as little or much control over what can be collected in your forms as you want. It would be possible to set all of your survey fields as TEXT data type fields, which would allow users to put any information in each field that they want. However, this may not be a good practice if you need a way to analyse that data in the future. Using data types restrictive to the information you want in each field ensures standardisation of your data which will help prevent errors and make your data easier to process and analyse.

In Mergin Maps, all vector layers and data tables that will be edited by users need to be stored as GeoPackage files. The GeoPackage file specifications in QGIS support the following data types that are commonly used for surveys:

TEXT

Variable length string data using UTF-8 or UTF-16 formatting. The data can be stored as single line or multiline. If field length is set to ‘0’ the field length will be variable, otherwise a character length can be specified. Text fields are also used for QGIS widgets for storing photos, QR codes and barcodes

INT, INTEGER

Integer (whole number)  data as INT32 or INT64 formatting. The user can also set a maximum character length for the field.

DOUBLE, REAL

A floating point number

DATE

A formatted text field that stores the date as YYYY-MM-DD

DATE/TIME

A formatted text field that stores the date and timestamp as YYYY-MM-DDTHH:MM:(SS.SSS)Z where T is a separator between date and time and Z refers to UTC timezone.

BOOLEAN,BOOL

A value representing true/false, stored as an INTEGER with values 0 for false and 1 for true. Used for the checkbox widget.

BLOB

A binary object of variable length. The length of the object (in bytes) can be specified by setting a max length or if it is unspecified, the objects stored can be of unlimited size. 

How will you store the data?

Mergin Maps provides users with a great amount of flexibility in terms of storing the project data. The simplest solution is to use the Mergin Maps Cloud, which is our own subscription-based premium SaaS storage solution that comes with customer support and routine service updates. The process to set up the service is simple and only requires a few steps to get started. Alternatively, you can host the service on your own servers by either using our fully supported Enterprise Edition or manage your own solution with the open-source Community Edition. 

Determining which of these solutions is best for your survey will depend on the size, expertise and resources available to your organization. We would recommend that first-time users take advantage of our one-month free trial to try out the Mergin Maps Cloud service as a way to familiarise themselves with the app and other tools available in the Mergin Maps ecosystem. 

Connect Mergin Maps to PostgreSQL and S3 

Mergin Maps also gives users the ability to connect their projects to external databases such as PostgreSQL and S3 with our db-sync and media-sync tools. The db-sync tool allows you to connect geopackage layers in your Mergin Maps project to a PostgreSQL database. You can also add an existing PostgreSQL table as a geopackage layer in the project. Once initialised, the tool will allow two-way synchronisation between the PostgreSQL layer and the project. 

How will you collect GPS data in the field survey?

You can collect your GPS location via cellphone with GPS locator or dedicated GNSS device. There are some differences and limitations to both methods:

Mobile device data collection

The biggest difference between a mobile phone and a dedicated GNSS device is the accuracy. The accuracy of GPS acquired through mobile phone is typically 3-5 meters. However, nowadays most of the mobile phones and tablets have built-in GPS sensors, resulting in no or just a little added costs if you want to collect data from the field.

GNSS device data collection

The main benefit of dedicated GNSS devices is high accuracy, typically within a few centimeters. However, the costs associated with GNSS devices are much higher, with prices typically in thousands of Dollars. Mergin Maps works with most of the GNSS devices on the market, here you can find a list of known and officially supported devices.

How to design a QGIS field survey?

Designing your survey in QGIS is one of the most crucial steps in preparing for field data collection. This phase determines not only what information you collect, but how efficiently it can be recorded, validated, and analyzed later. When designing your survey for field collection in Mergin Maps, there are a few things you need to consider. Once you have created the project and uploaded it to Mergin Maps, it is more difficult to change the schema of your project without causing errors. In particular, you need to be careful when adding or removing fields from tables in your project after it has been uploaded to Mergin Maps. Therefore, you may want to consider having a few extra text, integer, number or date fields in your table just in case you need to add something later. 

Data Schema

Start by creating a logical data schema, which refers to the structure of your data tables: layer names, field types, constraints, and relationships. Each field should have a clearly defined type (e.g., text, integer, date), with thought given to what values are acceptable and how those values relate across datasets.

Key recommendations:

• Use descriptive field names that are easy to understand by field workers.
  
• Limit free text input to fields where it’s absolutely necessary to avoid inconsistent data. Instead, try to take advantage of the value map and value relation widgets where appropriate.
  
• Define default values and use constraints like NOT NULL or value ranges to avoid empty or invalid submissions.
  

Survey Layers

All editable data in Mergin Maps must reside in GeoPackage format. This file format supports multiple vector layers, making it ideal for mobile surveys. Structure your layers according to the real-world features being recorded, for example, separate layers for infrastructure, vegetation, and land use. You can also store non-spatial layers in GeoPackage files.

Layers can also be grouped logically into themes within QGIS, which helps with styling, analysis, and field navigation.

Widgets and Form Layout

Widgets in QGIS allow you to customize how data is entered in your forms that appear on mobile devices. Selecting the best widget for a given field will help ensure data can be entered accurately and more efficiently by users in the field. Below is a list of widgets associated by the data type of the field:

Text

• Text Edit - free text entry which supports single or multiline text input. You can also enable support for HTML.
• Value Map - Create a drop-down list of values including the value to be stored in the table and description that will appear to the user. The list of values cannot be edited by the user and only one value can be selected at a time.
• Value Relation - This widget also creates a drop-down list of values that can be chosen by the user in the field. However, instead of a static list, the values available to the user are pulled from another table stored in the project. The widget can be set to enable selection of multiple values as well as the option to let users in the field add more values to the list of available values. You do not have to set up a table relation in the ‘Relations’ settings for this widget to work.

Numbers

• Range - Set a minimum and maximum range for values stored. You can also allow the user to enter the value with their device keyboard or clicking the ‘+/-’ buttons. You can also set an interval for the value increased or decreased by the buttons.
• Slider - Instead of editing the value in a box, the user selects a value by sliding a selector left or right to the desired value. You can set a minimum and maximum value as well as the interval to increase/decrease the value by moving the slider.

Boolean

• Checkbox - save true/false or other binary values by enabling a checkbox.

Date/Time

• Calendar - select the date with or without a timestamp using a calendar widget.

Photo

• Attachment - set values for filepaths of photos in your project. The path is stored as a text field linking to the attached file stored in your project. The mobile app enables the device camera or accesses the gallery to create and attach photos. 

Relations

• Configure 1-N relations to attach multiple records or photos to a single feature.
  

We recommend using the “Drag and Drop Designer” to design the layout of the form, which allows you to organise your form into tabs and group boxes which can optimise the layout for use on mobile devices. 

Constraints

Adding field constraints ensures data integrity. You can use constraints to ensure that surveyors record data that fits a desired format, does not leave a field blank, or even use conditional constraints to ensure conditions based on the value of another field. Several examples of constraints commonly used are:

• Define expressions to limit values (e.g., only allow numbers between 0 and 100).
  
• Use expressions like length("name") > 3 to enforce text rules.
  
• Use the NOT NULL flag to prevent saving records with missing data.
  
• Use the Unique constraint to ensure the value is unique compared to values recorded for other features
  

Table Relations (1-N)

Complex surveys often need to record multiple entries for a single feature, for example, recording multiple maintenance events for a single pipeline or attaching multiple photos to a single feature. This is handled using 1-N relations in QGIS. We would recommend following our documentation for setting up 1-N relations in your Mergin Maps project.

Spatial Joins

While spatial joins are not directly editable in Mergin Maps, they can be used pre-survey to enrich your data, for example, automatically assigning administrative boundaries or environmental zones to point features. This step helps reduce manual input and improves accuracy.

Managing Photos

Mergin Maps supports photo capture and attachment to features during survey work. Several things you will want to consider when setting up your Mergin Maps project are how you wish to store your photos, the quality of images required for your project and whether you want to attach multiple photos to individual features. 

Photo Storage

By default, photos captured by the mobile app are stored in the project folder. However, you can also specify subfolders within the project folder to store photos linked to your project. Mergin Maps also allows you to:

• Set an expression to automatically name photo files with a custom filename
  
• Use selective synchronisation so individual users only download photos attached to the features they record in the field
  
• Resize photos to a maximum quality setting to limit the overall size of the project storage
  
• Use value expressions to record EXIF metadata such as location, orientation or timestamp
  

You can find more information about photo management in your Mergin Maps project by reading the documentation.

Multiple Photos per Feature

Mergin Maps also supports attaching multiple photos to a feature by using 1-N relations. You can find steps on how to enable this, along with an example project, in our documentation.

External storage with S3 and Google Drive

If you have a lot of photos in your project and want to save storage space on your Mergin Maps workspace, Premium users have the option to use the Media Sync tool. This tool allows you to synchronise media files from your Mergin Maps project to other storage backends such as Amazon S3, MinIO and Google Drive. You can select to either backup your media storage or move media files from your Mergin Maps project to your desired storage backend.

How to Use QGIS Background Maps in Mergin Maps for Field Surveys

Background maps are an essential reference for workers surveying in the field. An appropriate background map will give the user the information needed to determine the location of the feature they are recording in reference to their surroundings. Mergin Maps supports the use of online and offline background maps. 

The type of background map needed will depend on the availability of internet connection for your surveyors. Offline background maps can use a lot of storage space if you are surveying large areas, but users will need a stable internet connection if they are relying on online background maps. Therefore you will need to consider the most appropriate option for your project. You can find more detailed information about selecting and managing background maps for your project in our documentation.

External Data and Themes

Preloaded reference datasets, such as infrastructure, land use, or boundaries, can be added as read-only layers to help with field orientation or validation.

Once you have collected your background maps and reference data, you may wish to organise different themes for workers in the field. This can be a good way to allow users to reduce clutter in the project interface and only view layers necessary for the task they are working on. 

How to Use QGIS Symbology

Creating effective map styling in QGIS before deploying the project ensures field users can easily interpret what they’re looking at. Mergin Maps supports multiple symbology methods available in QGIS:

Simplified

Use basic symbology for simple maps with low cognitive load, ideal for beginner users or minimal interfaces.

Categorized

Apply categorized symbology to distinguish features based on an attribute, for example, different species types, condition grades, or land use categories.

Rule-Based

For more complex workflows, use rule-based symbology to apply multiple layers of logic, for example, show a feature in red if its inspection date is over a year old and in green if recently checked. It is important to note that rule-based symbology is more memory-intensive than other methods. Therefore, if you have a very large project you may experience slower performance in the app. 

Labels

QGIS label settings are also respected in the Mergin Maps app. Use expressions to dynamically label features based on one or more attribute fields. Keep labels short and legible.

Custom symbols

Mergin Maps also supports the use of custom SVG symbols in your project. If you wish to use custom symbols, you will need to ensure they are uploaded to the project directory. 

How to Collect the Data in QGIS

Once your project is fully prepared and published to Mergin Maps, the data collection phase begins. This is where all your preparation pays off, QGIS and Mergin Maps work together to offer a seamless field experience.

Uploading the Project to Mergin Maps

Uploading your project prepares it for mobile use. Once the project is uploaded to the Mergin Maps server, it can be downloaded by users logged in to the mobile app and they can begin to collect data. Before uploading the project to Mergin Maps, you will want to ensure all layers that will be edited are in the correct GeoPackage format. When the project is ready, you can follow our documentation to upload the project depending on the method you wish to use.

Project Management

Once your project is uploaded, you will want to invite users to access the project if you are working collaboratively. To do this, you will need to log in to the Mergin Maps Dashboard. Here you can manage your workspace. Think of the workspace as a place to manage projects, users and subscriptions in Mergin Maps. You can see an overview of your projects, webmaps and grant access to your projects. 

There are two ways to add users to your projects. You can add users as workspace members, which will give them access to all projects on your workspace, or you can give them access to specific projects by adding them as guests. Members and guests can also have different roles. Workspace member roles are applied to all projects they have access to, while guest roles can be defined on a per-project basis. The different roles available are reader, editor, writer, admin and owner. Most users who are active contributors to the project will only need editor or writer access. Editors are only able to add, edit or delete features on project survey layers while writers can also make changes to project and layer settings in QGIS. You can find more information about user roles in our documentation.

Updates & Error Fixing

After your project is deployed, it may need revisions. You can make changes in QGIS, such as adjusting form widgets, changing symbology or adding new layers and then push updates to the cloud. However, you will want to be careful if you are making changes to your data schema by adding or removing fields on existing project tables. If you are making updates to the data schema, you will want to be aware of our recommended best practices.

Mergin Maps App

The mobile app is where users collect data directly. Available for Android and iOS, the app provides intuitive interfaces and offline capability for work in remote locations. Below is a brief overview of the app interface, but you can find more information and screenshots of how to navigate the app in our documentation.

Interface Overview

The main sections of the Mergin Maps app include:

• Projects Tab: Browse or search for your assigned projects.
  
  
• Map View: Navigate to features and collect new data.
  
  
• Forms: Automatically generated from your QGIS layer schema and widget settings.
  
  
• Sync button: Upload and download data changes with a single tap.
  

Users can also view background layers, use GPS to zoom to their location, and toggle between base maps.

Downloading a Project

To start data collection:

1. Open the app and log in.
   
2. Navigate to the Projects tab.
   
3. Download the assigned project to your device.
   
4. Ensure all required media files (e.g., photos, MBTiles) are downloaded by waiting for the full sync to complete.
   
5. Wait for the map to load, once complete, it’s ready for use.

Collecting and Editing Features

Using the app, users can:

• Tap the "+" button to create new features.
  
• Fill out the form using dropdowns, checkboxes, and camera inputs.
  
• Use GPS to set feature geometry (points/lines/polygons) or draw manually.
  
• Attach photos using the Photo widget.
  

Features are saved locally and added to the synchronization queue.

Synchronization

Mergin Maps uses Mergin Maps Cloud to manage synchronisation between users:

• Changes are tracked locally.
  
• Sync occurs when connected to the internet.
  
• Only the changes and new attachments are uploaded, minimizing data usage.
  
• Synchronisations from different users are rationalised based on project information. If there is conflicting data synchronised by different users, a conflict file will be produced and added to the project directory which includes which information was edited by different users.
  
• It is recommended that users sync as often as possible to avoid data conflicts and errors.

Users can sync:

• Manually via the Sync button.
  
• Automatically every time a change or new feature is saved.
  

Project Completion

Once field collection is complete, you will want to ensure that all users synchronise any final changes from their devices before synchronising your project in QGIS to pull all of the changes from the cloud to your local version. Now you are ready to review edits, validate data, and finalize the dataset.

Project history is preserved, so you can access previous versions if needed. This is particularly useful if data was mistakenly edited or deleted. There is also a project history tool in the QGIS Plugin that allows you to visualise changes made in previous versions of the project. You can also go back and add changes from a previous version to the current version of the project, which may be useful if there is a data conflict or some features are accidentally deleted.

Reporting Results

Once data is synchronized and validated, you can move on to generating reports and performing analysis in QGIS. You can use attribute tables and summary statistics to explore patterns in the data by performing spatial analysis such as proximity, overlays and clustering. You can also generate heatmaps or charts based off of information collected in the attribute forms. If your features are timestamped, you can also use other plugins like Time Manager to analyse time-series data in your project. 

Once your analysis is complete, you can use the QGIS print layouts to produce maps and reports of your project and findings. You can also export your data in different formats such as CSV, PDF or GeoJSON to connect to other programs or datasets for further analysis.

Conclusion

Planning and executing a field survey with QGIS and Mergin Maps gives you a complete, open-source solution that combines flexibility, precision and scalability. From project design to real-time collaboration and data analysis, this workflow supports professionals across industries, whether in environmental monitoring, infrastructure, urban planning, or research.

By investing time in clear project design, setting up robust forms and constraints, and leveraging tools like versioned sync and offline maps, you set your field team up for success, and ensure the data you collect is accurate, consistent, and ready for action.

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