Linear Referencing of Longitudinal Survey Data
by Ethan Welty, 01.28.2008
Software: ESRI ArcGIS 9.1, Microsoft Excel
(An ArcGIS Desktop license is sufficient for this protocol)
1. Prepare the initial survey data for analysis.
a) Append all the data collected for each survey unit into a single table. GPS coordinates
[GPSX]&[GPSY] and unit length [LENGTH] are required fields for Linear Referencing. Unit
lengths of zero are allowed, but require extra housekeeping later (see Part.4d). The GPS
coordinates can either represent the start or end point of a unit, but this designation must be
consistent throughout the dataset.
b) Arrange the rows of the table so that the units are in the order in which they occur along the
survey path and create a new index field [UID] so that this ordering can be preserved.
c) Create a new integer field [RID] whose value matches the ID of the ArcGIS route to which the
data will be projected. The route should be an accurate trace of the survey path and have M
values increasing in the same direction as the units are arranged in the table.
d) Add two new fields [FROM]&[TO] for the position of the unit endpoints. The first unit's
[FROM] is assigned a value corresponding to the starting position of the survey. Calculate the
remaining values such that [TO] = [FROM] + [LENGTH] and [TO] carries over as the [FROM]
for the following, adjacent unit. If there is a known gap to the beginning of the next unit, it should
be accounted for in an increase in the following unit's [FROM]. Any units off the main survey
path (ie. side channels) can be included, without affecting the placement of other units, by
assigning to them a non-length, that is [FROM] = [TO].
2. Import the prepared data into ArcMap.
a) Create a point layer of the GPS waypoints by pointing the “Add X, Y” tool to the field-acquired
GPS coordinates [GPSX]&[GPSY] in the prepared table.
b) Add the survey route to the map.
c) Create a polyline layer of unit segments by using the "Add Route Events" tool and specifying the
survey route, the unit endpoints [FROM]&[TO] and the route ID [RID].
d) Adjust the symbology of the polyline layer by applying a color gradient to unique values of the
[UID] field so that every unit is distinct. You will likely notice that the GPS points and the
linearly referenced data are not in agreement. This is to be expected, since the former relies on
absolute positioning, while the latter relies solely on estimated unit (and gap) lengths.
3. Create anchors from the GPS waypoints.
a) Right-click the point layer and select ‘Export Data…’ to save the data to a point shapefile. Agree
when it asks to add it to the map, and hide the original point layer to avoid confusion.
b) Start an edit session for the point shapefile. Set the snapping environment so that the points can
be snapped to the survey route. Carefully consider unit-specific field notes, landmark locations
(ie. tributaries, roads, structures), the dynamically segmented data and the original GPS locations
when choosing where to place the points along the channel. Keep only points whose positions are
certain or near certain, or that delineate units whose lengths must be kept fixed (often important
for short units, which can become squished when bounded by long units); the others can be
deleted. This allows for more degrees of freedom during the calibration process, leaving it up to
the computer to find the best fit for the segmentation. Save your edits and close the edit session.
4. Calibrate the linearly referenced data.
a) With the anchor points in place, run the “Calibrate Routes” tool on the survey route to create a
new route with warped (calibrated) measures. Specify the anchor points as the input point
features and either [FROM] or [TO] as the measure field, depending on whether the GPS
waypoints represented the start or end point of a unit. Uncheck the option to “ignore spatial gaps”
to preserve any gaps between survey units. The calibrated survey route will be automatically
added to the map.
b) Run the “Add Route Events” tool on the table prepared in Part.1 (see Part.2c), this time using the
calibrated survey route as route reference.
c) Adjust the symbology of the new polyline layer so that every unit is distinct (see Part.2d). If
calibration worked properly, the resulting unit endpoints should fall exactly on the corresponding
anchor points. All that remains is transforming the route measures ([FROM]&[TO]) to map to the
uncalibrated route.
d) Run the “Transform Route Events” tool on the newly created unit segments, with the calibrated
survey route as the ‘source route’ and the original survey route as the ‘target route’. The
[FROM]&[TO] values of the output table will reflect the measure system of the original survey
route. Unfortunately, any zero-length units will be removed by this script – they must be added
back into the resulting table with manually updated [FROM]&[TO] values.
e) Run the “Add Route Events” tool on the output table, using the original survey route as route
reference. Linear Referencing allows changes of [FROM] or [TO] values in the table to be
displayed dynamically on the map. The spatial representation of the data relies on the relationship
between the table and the survey route.
f) If a standalone file is necessary, right-click the events layer from above and select ‘Export
Data…’ to save the data to a point shapefile or geodatabase feature class.